Hybrid Hydraulic System

A hybrid hydraulic vehicle (HHV) is one that typically includes an internal combustion engine (ICE) which is coupled to a hydraulic pump which powers a hydraulic pump motor. Energy is stored in one or more accumulators in the form of a compressed gas, typically nitrogen. The vehicle is propelled when the pump motor is provided hydraulic power either directly from the pump motor or from the accumulator. Air in the accumulator is compressed by the hydraulic pump or by the pump motor during regenerative braking.

The two main advantages of a hybrid hydraulic system are: 1. It allows for power from a small ICE to be combined with that of an accumulator to provide power for short periods of time equal to that of a much larger engine. Because the small engine in a HHV is not coupled directly to the drivetrain, it can be run with its throttle fully open at its speed of optimum efficiency at all times. In a conventional vehicle with a larger engine that provides the same acceleration, the engine must run at speeds proportional to gear ratios in a transmission and rarely runs at peak efficiency. 2. Kinetic energy from the vehicle can be converted to stored energy in the accumulator during via regenerative braking when the pump motor is run in reverse an takes energy from the wheels and stores it in the accumulator.

A series HHV is currently being considered for the Open Source Car. Animations explaining how a series HHV works are provided by the EPA here: http://www.epa.gov/oms/technology/research/how-it-works.htm --Crank 18:23, 24 April 2011 (PDT)

Math
A hydraulic hub motor (no gearing) would need to put out 1,000 RPM, with a 24" diameter tire, to go 70 mph.

Active Hydraulic Suspensions
Lotus prototyped active hydraulics in its F1 racecars. While it did improve cornering speed, the hydraulic pump consumed horse power. The load is estimated at around 4.5 to 9 hp depending on the smoothness of the road. Lotus Engineering used the Esprit as its commercial development platform. It got the hydraulics from the aerospace industry but discovered that it was the software that was the biggest bottleneck. By the 3rd and 4th generation of the system backup coil springs were included to support vehicle weight in case of system failure.

Active suspensions have been banned from grand prix by the FIA. Cars equipped with them were beginning to dominate the race, forcing every team that didn't have the tech to spend a lot of money to catch up. Additionally, because they allowed higher speeds over rougher ground, system failure resulted in exceptionally destructive crashes. Former Benetton mechanic Steve Matchett makes several references in his books to his dislike of working with hydraulics that threatened to fire hot oil at you at a pressure of 2,500psi if a mechanic disconnected the wrong component at the wrong time.

=Real Examples=

Commercial Solutions
This is a steerable hydraulic wheel motor by Poclain. It combines two hydraulic motors into one unit such that one turns the wheel and the other steers and the whole thing just bolts onto a flat surface. Its max RPM is 170...so...not a speed demon, but max torque is 6500 lbft. Maybe one can be made that reverses that proportion. At any rate it would work for a tractor.

Lighting Hybrids has a retrofit for fleet trucks that is basically a hydraulic motor/generator and an accumulator. It captures braking energy and returns it during acceleration. They claim a 40% improvement in fuel economy.

DIY Solutions
This is a summary of the project Ruckus from hydraulicinnovations.com wrote up in the forum.

(1.171cu.in.) Each eats about 18gpm @ 3500rpm. About 46 ft.lbs. of torque each @ 3000psi. The Talon Diffs are 3.545 which gets me to about 73mph on the stock tires. 1. TURN THE ENGINE OFF! which is the approach taken by Carman (using a large motor with an accumulator. or 2. REDUCE ENGINE SIZE! which was done in part by Ernie Parker (also with an accumulator). The hydrostatic system allows me to reduce the engine further than a traditional transmission since it allows tiny "gear" shifts. If you look at the power to weight ratio of a laden semi truck it is very low. But they run and drive ok because they have LOTS of gears. Hydrostatic is infinite gears, so it will allow me max performance from minimum motor - as long as I keep the system simple and minimize flow-impeding valves, bends, etc. So I plan to make a VERY simple closed circuit system (yes, with a charge circuit, tank, and cooler (if necessary)). Then I will add more complexities such as regen braking and see if the added benefit outweighs the costs. The only way to get more than 300ftlbs is to run at a higher pressure, run larger motors, or decrease tire diameter. If I run larger motors then I cannot reach max speed (70-75mph) without overspeeding or upsizing my pump. So I have two sources, one using hp, the other tq, and I get very similar results... There will be 'cogging' when running in series. Whether this will be serious or not, I don't know. Consider feeding the front wheels first, output of that motor to the rear wheels. Controlling rpm for both a variable power source and a variable pump will be interesting, especially in traffic. I can also add another auxiliary motor to the rear diff and another accumulator if I wanted. Since the aux power circuits are totally separate, they can be different motors, different psi, different size... imaging having 2 boost buttons, one a long-burn, one a fast-burn, or use both together...
 * Pump rpm too low for gas motors. I finally found a 3600 max rpm pump which can be mated to a ~3600 max rpm industrial engine. (note- I ended up using a redrive)
 * Finding a Motor with the correct rpm and tq ratings. I found I had 2 viable options. The first is to use 4 motors, one at each wheel. This configuration requires ~1000rpm to achieve 70-75mph (my target speed) depending on tire size. This is still an option, but I could not find 4 motors at a decent price that met my required specs. The other option was 2 motors, each driving a standard differential. This required a max rpm of about 3-4,000. Again, depending on the the differential gearing and tire size.
 * Project Vehicle= 1990 Eagle Talon Tsi AWD
 * Pump= 0-1.24 cu.in. variable displacement piston pump with an integral charge pump, mechanical swash control, and a through-shaft so I can mount another small pump or accessories. At 3600rpm this pump puts out a max of ~18gpm and has a max pressure of 3,000psi.
 * Motors=Sauer Danfoss snm2 fandrive motors. Fixed displacement 19.2cc
 * Accumulator= 5gal. Parker 3000psi bladder...It seems a good accumulator is REALLy pricey.
 * the Parker car was reported to do ~70mph on ~16hp, I'm thinking of going with about 18-24hp to be safe...electric AND manual start.
 * Ok, my aero drag and rolling resistance calculations suggest the car will require ~16hp to go ~72mph. At 3600rpm the pump requires 16hp to produce 1500psi. and the motors put out, you guessed it, 16hp at 1500psi at 3500rpm. So, I THINK my components are appropriately matched.
 * From Justajo: The main reason - apart from lack of money to finish - that I had to stop my project? State air-quality regulations...I would suggest that before you get any further into your project, you check on how your state or your local area views replacing the stock "EPA-approved" engine that is in your donor car now with an engine that is not so approved. What Carman and Ernie Parker did way back in the good ol' days before the massive governmental intervention that we have today, is currently not possible in most areas of the country. In the Phoenix metropolitan area, Maricopa County, and in certain adjoining communities (where I live), it is not legal to swap engines the way in which you want to do it.
 * From Ruckus: I think all you need is a "pre-smog" vehicle (-1975ish). Even in California vehicles built before smog regulations do not have to meet any. If the requirement is merely for a DOT-approved engine, then use a motorcycle engine or a tiny geo, suzuki, old subaru, etc...As for motorcycles, Believe it or not, if you weld the rear suspension solid or mix and match motors/frames it becomes "custom-built" (aka chopper) and NOT a "motorcycle". As such, it needs no extra safety equipment such as say, a muffler, blinkers, headlight, speedo, brakelights, or get this, BRAKES! (in Montana)
 * The reason for the VW only achieving ~70mph is hidden in your data - a cd of .48! To get real-world hp you multiply the cd by the frontal area (.48cd x 19.37sqft = 9.3cda) then, use the formula cda times the cube of the velocity and divide the result by 150,000. So... (9.3 x 357,911)/150,000 = 22.2hp just used to push air at 71mph. Add 10-15% for mechanical conveyance and ~6hp for rolling resistance to get 31-32hp. Remember the vw's hp was "gross" which does not include alternator, muffler, or air filter. So this calculation seems about right. Now, the Eagle Talon has a cd of .29 x a similar frontal area of 20.17sqft x71mph cubed divided by 150,000 = 13.96hp to move air at 71mph. It's amazing how much a slightly better cda affects hp requirements at high speed. If I slow the Talon to 65mph it only needs 10.7hp to push air. So, I plan to build the thing with 18-20hp and then aero-mod the vehicle as needed to achieve my desired top speed. I will also be running narrow tires pumped quite firmly. Also, weight affects acceleration much more than top speed. And don't forget those old bias ply tires had a lot more drag.
 * my biggest design compromise is requiring a specific top speed. If I was willing to slow down, I could get fantastically better mileage. What the hydraulic system WILL allow me to do is get ~normal acceleration from the stored energy in the accumulator. In other words, I am building a very "anemic" vehicle with enough "boost" to drive normally. The corner hp on my pump (max gpm AND max psi) is about 35 hp. So I can just upgrade the engine if I find the performance lacking.
 * As for the AWD, I wanted that for 2 reasons. The first is I live in MT and have to commute 66 miles a day whether it is solid Ice or slush or whatever. The second is I had a hard time finding a motor that was large enough to output a reasonable amount of torque (~100ft.lbs.) and still go 3500rpm. Sure, they exist, but at what cost? I came across 2 motors that were brand new, rated for 3500rpm, were small enough that my pump could supply them all the way to 3500rpm, and make 93ftlbs together at 3000psi. The clincher is they were $60 each!
 * As for needing 50hp and less weight, That, again, is what the accumulator is for. The Suzuki you mention has 58ftlbs of torque. 1600lbs/58ftlbs=28lbs/tqftlb. My hydro motors are rated at 93ftlbs. Just for fun, lets assume my completed project weighs the same as the stock Talon (I think it will weigh less). 2500/93=27lbs/tqftlb. So they are just about equal.
 * Will I be smoking Corvettes? no Should it accelerate just fine, yes. Provided I can produce 3,000psi. That is why I bought a tank. I am hoping to charge it with regenerative braking or even while I coast to a stop. THAT is the whole reason for going hydro, energy storage which can be released as "boost" to make a totally anemic vehicle driveable. At 3000rpm my motors will use ~15gpm. A 5 gal tank will supply ~10-15 seconds of "boost". Next time you speed up to pass somebody, count how long you acellerate. 10 seconds is a LONG time. Once the car is going, it has the hp to keep going. It is just the acceleration that is the hard part
 * The bottom line, according to my calculations, is that fuel savings are going to come mostly from REDUCING THE SIZE OF THE ENGINE. A good example of this is the initial Parker prototype which did little better than stock using a hydraulic transmission. It was changing the engine which gained the savings. A hydraulic energy storage system allows an engine to be sized for average hp instead of max hp. In my mind, bigger and higher pressure tanks should account for a heavier vehicle.
 * If the Parker car only had 2-3 gallons of accumulator, increasing that would be a great way to improve the design, especially if applying it to a heavier vehicle. One large accumulator is much more efficient in terms of gal/lb than several small ones. But ultimately, the more storage, the better. Until weight becomes a problem.
 * Just one of my Talon differentials weighs more than my pump and motors combined. When you are talking about Dana 60's and 80's in trucks, the weight and lost energy is unreal. 4 hydraulic wheel motors can take a tremendous amount of weight off the chassis. I have a NP205 transfer case in my garage that is HEAVIER than my 5 gal accumulator.
 * I decided to go with differentials for 2 reasons: 1. I found a deal on two fixed gear motors that met my (theoretical) requirements before I found a similar deal on 4 wheel motors. 2. If I end up not liking a fixed motor and want to go to variable, I will only have to find 2, not 4. Heck, I could even ditch the AWD concept and make it just a fwd or rwd if I wanted to and only had one motor.
 * I need to redesign using parallel motor circuits. Assuming I use the same 1.24cuin pump rated at 17gpm@3600rpm, I would need to resize the motors to about .55cuin to run in parallel since they would use twice as much fluid.
 * Top 10 reasons for running gas: 1. Easy Cold starts. 2. Nothing like impressing chicks with "I drive a hybrid" only to have a big puff of smoke come out when you start it up. 3. THAT smell... ugg. (see #5)4. $$$$$$$$$ - why are they soooo expensive anyway? 5. While it smells less, I can't support biodiesel until it is made from non-food products like hemp. Making fuel from corn is like burning furniture to heat your house... 6. Size, in-line Diesels tend to be taller than a horizontal opposed or v-twin gas. (ok, this doesn't seem a very good reason, but my engine bay is low) 7. WHAT?? I can't hear you over that clanking sound.... I prefer engines that don't sound like a dryer full of lugnuts... 8. Diesels run reliably until they stop, then, good luck... Gasers take more tinkering, but when they stop you can usually get em goin again. 9. Diesels mostly seem more efficient because a gallon of diesel is heavier (7vs 6.2 lbs) and thus contains more energy. If you compare btu's per pound, gas actually has more. In Montana Diesel is SO much more expensive that the extra efficiency per gallon is lost. 10. Did I mention Cold Starts? folks on this site are running very nice modern mini diesels which don't have many of the problems I listed above. If I had the cash, I would consider one, but they are just way out of my budget for this project.
 * From AaronD: - You start the vehicle up and the entire system (including the accumulator) charges up to your regular running pressure (3000psi). - You drive away in parallel 2 motor (4wd) mode with 93 ft-lbs of torque. But it's using a lot of CFM as speed builds, so you "shift" into series motor mode at about 35mph. - Now your torque available drops to half, but your top speed doubles with the same CFM. But that's just for the initial start. After you're up and running a stop/go looks like this. - Press the brake and go into regen mode. Measure the distance it takes you to slow from 70 to zero and calculate the amount of fluid the pumps push in that distance. Size the accumulator and precharge so that adding that amount of fluid to it raises the accumulator pressure to near it's max pressure (say 4500psi?) - Now when you press the accelerator to take off from start you have about 140ftlbs of torque available because you're starting out at 4500psi instead of 3000. The torque tapers off as the accumulator drains and pressure drops back to the normal running torque at the 3000psi that the motor provides. Remember that the whole purpose of using an accumulator is two fold. 1) It can store and later supply volume to the system over and above what the pump provides. So if you need 3000 psi but double the CFM that the pump provides during parallel motor accel mode, the accumulator provides that fluid. 2) It can store fluid at a higher pressure than the pump can provide if it's charged from the motors during regen braking. This effectively increases your available torque during acceleration. So just like you're planning, you can size your engine for the average needs of the system (at cruise speed) and when it needs more CFM for parallel motor mode, or more pressure (for faster acceleration) than the pump provides at that RPM, the accumulator can come in to fill the need for short bursts. "Short" being relative of course, but your guess of 10-15 seconds is DEFINATELY long enough for anything you do in an average car.
 * Ok, so here's the rub -> I originally envisioned using an accumulator to store regenerative braking energy and to store energy normally lost during idling or times of low energy consumption (coasting). Unfortunately, accumulators seem to work easiest with an open-center system. This would mean a fixed displacement pump charging the accumulator. A valve would open to release the pressure/volume to the motor as needed. In some designs the motor shuts off and restarts as needed to keep the accumulator within a certain pressure range. The main benefit of this type of design is you run the motor at peak efficiency. But this design works best at making a big motor run efficient. In my case, I am gaining significant efficiency by reducing the size of the engine, so small gains from braking regen etc. may not be worth the increased complexity (cost) and parasitic losses of an open-circuit design. The other issue is I can't reach my desired top speed (70-75) unless I run fairly small motors (3500rpm at diff) or spend big money. Likewise, my pump is sized to barely get those motors up to speed (17gpm). Because I have already purchased a variable pump and fixed motors that are properly sized for my desired rpm etc, I am thinking I will build a closed-loop system without an accumulator first since it is much simpler. Since my motors are fairly small, I am thinking valves to switch between series and parallel would be wise, although again, cost and complexity rise substantially.
 * Yes, Carman had valves on the motor circuits because he was running open circuit systems. He was basically making a car run like an air compressor, decoupling energy creation/storage from its periodic use. This works best for city driving. I wish they had done a highway test as well.
 * My research has shown that there are 2 key ways to get high mpg.
 * From AaronD: If you use an accumulator to store pressure above the normal system running pressure instead of just CFM at the same pressure, then you have access to higher torque than the motor could ever make on its own at times when you need it (like acceleration). So for example, you take a small 100hp car and convert this way. First, you downsize the engine to a small and efficient 20hp version, and choose a pump that delivers 2000psi with just enough CFM to maintain your desired top speed. Obviously, with 20hp and the torque available there your acceleration will be pretty dismal. But if you add an accumulator and rate all your other components for 4000psi here's what you can do. When you step on the brakes you store all the flow of the motors in the accumulator which is precharged so that after you stop from 70mph it has charged up to 4000psi. Now when you leave the stop sign you switch to parallel drive on the motors (double torque, half top speed). Now instead of 2000psi at the motors, you have 4000psi running in parallel which quadruples your total available torque. So the car now leaves the stop sign as if it had 80hp, instead of 20.
 * Yes, motor torque is determined by motor displacement and max pressure. Running higher pressure in your accumulator/motor circuit Would be a way to increase available torque, but it is unneccesary. I also think trying to run a system on 2 different pressures could invite trouble. My calculations suggest my current motors (1.17cu in x 3000psi) produce "just enough" torque (~155ftlbs at the ground) to drive around in SERIES. The rig would be gutless, which is ok, but I worry it might have difficulty pulling out into traffic or fail to produce enough starting/low end torque on a steep driveway or in mud/snow. Just switching to PARALLEL gives me DOUBLE the torque (~315 ft lbs at the ground). This would give it a similar power to weight ratio as your typical econobox. This seems plenty powerful for a commuter car.
 * Yes, many drivers coast in gear which is using engine compression as the brake. But you can also put it in neutral or push in the clutch to gain additional "coast". This just means you stop "gassing" the motor even sooner. It would be no different in a hydraulic setup. I was planning to put a "clutch" valve on my the old clutch pedal. This would create a bypass around the pump and allow "free-wheeling". My right foot pedal would control the pump swash and if let up would create hydraulic braking via the motors. Since they are fairly small, when shifted into series this would not produce a "screeching halt", but be somewhat similar to letting off the gas in high gear in a manual transmission.
 * Yes, I only need 16-20hp on the highway, depending a bit on speed, winds, grade, tire pressure, aerodynamics, etc. That is the horsepower requirement to which I am sizing my engine. You ask if my engine will not be able to maintain "normal running pressure" at high speed. I will be able to produce about 1500psi at full flow (17-18gpm). This is the same as saying "20hp". -But I cannot produce 3000psi at full flow because that would be a whopping 37hp. This is "corner hp" and is the maximum energy your pump can transfer. If you size your engine to corner hp then it is overkill in every other situation but full throttle at top speed - much like a traditional car has an engine sized for the "worst case scenario" which is wasteful the rest of the time.
 * The hydraulic motors' displacement limits available torque, yet produce plenty for "normal" driving when in parallel. An accumulator would only create additional torque by adding addional pressure/flow above 50mph where my engine starts to be able to produce less than the max pressure because it is supplying max flow instead.
 * Since I theorize that the largest gains can be made by downsizing the engine and being happy with a vehicle that accelerates "normally", my first "phase" will be to produce such a system. Accumulators are VERY expensive. They are also heavy and have specific mounting needs (bladder accumulators should be mounted vertically), and some people worry about explosion in a crash. The small increase in efficiency might not be worth the added cost, weight, and complexity of a regen system.
 * An open system is set to a predetermined pressure, say 3000psi. This means that when you are sitting at a light or cruising in town your engine is pushing HARD against the pump in order to create that 3000psi. This is a constant energy drain. Most sources say these types of circuits are typically 20-40% LESS efficient than the same setup with a variable piston motor. The more time you spend idling, the worse an open system is. By comparison, a closed system only produces the pressure NECESSARY to move the motors. Cruising in town this is VERY LOW (250-500psi, or about 2-4hp). Thus it greatly increases efficiency during city driving. The open circuit would ALWAY be using a full 20hp -Even when only 2 is needed!
 * In a complicated hydraulic system, I would think a starter circuit would be just as reliable as any other part of the system. But again, the devil is in the details. Electric starters are most prone to fail due to the BS engagement mechanism. If you had an electric motor with a belt-driven electric clutch like an air-conditioning compressor, then the mechanism would likely be more reliable. One advantage with hydraulics is the starting speed wouldn't be as succeptable to cold weather. You might get a lot of cranking out of a large accumulator. But once it was empty - then what? Would you have to tow the vehicle in hydraulic brake mode to "recharge" the accumulator?
 * From AaronD: I'd be packin' a small handpump to build up enough starting pressure for that kinda situation... maybe work it into the brake pedal... it'd give ya a new reason to pump the brakes!...I kinda like the idea of restarting the ICE with the pump... and agree there must be a simple way to plumb that in as an option... especially with a fixed displacement pump... aside from a valve or two, it seems like everything else would already be there, so little to no added weight...
 * From AaronD: After reading through your description again I think that you're effectively doing the same thing with your engine and pump sizing as I was suggesting with the accumulator. It's so small that it can only provide for example 1500psi at max flow for your top speed, but at low speed can deliver 3000psi. That's the same effect as charging the accumulator from braking energy to 3000psi while using a pump that can only supply 1500psi over your entire flow range. Like everything else in hydraulics, I think we're basically talking about doing the same thing in a different way. Yeah, I forgot about not wanting to go over ~3000psi. Having a 1500/3000psi system is much less expensive. Don't forget though, all manufacturer hybrids on the market today get their gains from boosting a small engine at low speed with energy gained when braking or otherwise excess, and turning off the engine at idle. An accumulator makes those jobs possible with a hydraulic system too, and is 4-5x better at the regen portion than an electric motor with batteries. So as much as you'll be able to improve mileage with just the small engine, everyone who makes money selling hybrids thinks that it's worth it to regen/boost and engine off too.
 * 3000psi seems pretty standard to me. Used and surplus pumps rated to ~3000psi are plentiful...If I had a heavy car, a hilly commute, and lots of stop and go traffic, then regen would be high on my list. I want to start with a simple, minimalist system and then work up from there (regen, larger pumps, larger motors, fancy schmancy circuits, etc.).
 * From mudguard: Automotive parts supplier Eaton is best known for its forced induction systems, but now the company has unveiled details behind a product that it claims can improve fuel economy by "between 50 and 70%". Known as the 'series hybrid hydraulic system', Eaton has revealed the new technology is being developed jointly with the Environmental Protection Agency under a Cooperative Research and Development agreement, giving the company some credibility in its claims...The vehicle uses hydraulic pump/motors and hydraulic storage tanks to recover and store energy, similar to what is done with electric motors and batteries in hybrid electric vehicles. In a series hybrid hydraulic system, Eaton claims that fuel economy improvements of between 50 and 70% are achieved in three ways - through utilization of braking energy that normally is wasted, more efficient engine operation and a stop/start system for the engine when it is not in use.
 * Posts of another member on the forum led me to investigate military surplus engines. All I can say is "WOW", how did I miss that? There are 3 Teledyne military-spec engines which folks may find of interest: 4a032 4cyl 32cuin ~17hp ~20ftlbs 1.4 gal/hr max; 2ao42 2cyl 42cuin ~22hp ~32ftlbs 2 gal/hr max; 4a084 4cyl 84cuin ~44hp ~61ftlbs 4 gal/hr max; These are 4-stroke, air-cooled, and yes, beautiful to behold
 * The weakest part of my system is the low-gear torque value. With both motors at 3000psi I will get only ~320ftlbs MAX at the ground. According to torque calcs, this will power a 3000lb vehicle up a 10% grade and accelerate at 2mph/sec or 60mph in 30sec. Since my car will likely be lighter, I am expecting a max grade of ~12% and a max accelleration of about 2.25mph/sec or 0-60 in 26 seconds.
 * ...even diesels waste about 50% of their fuel at 20% load. Optimal loading is 80-90% for diesels and 90-100% for gas. So maximum efficiency is achieved by selecting a motor that is "worked" pretty heavy. Kind of like the old volkswagen buses where you plant your foot on the floor and steer. This is why I chose a 20-22hp motor for an estimated 16-18hp of use. This puts me right at 80-90% load at 70mph. Of course, acceleration will likely use 100% load where gas slightly out-performs diesel.
 * Shifting from series to parallel doubles the pressure and motor torque, creating a true 2-speed "transmission" where torque is multiplied. A variable displacement motor can also produce variable torque by changing the ratio of pump to motor displacement and thus is more like a true "transmission". But good luck finding them cheap...The issue I see with inexpensive fixed displacement motors (like I plan to use) is if you select a motor with decent torque, then it gobbles insane gpm at speed. An efficient hydraulic hot rod would have a small engine, large pump, large accumulator, and large motors. The large motors and tank would allow rapid acceleration while the small engine would get good gas mileage. The reason for the large pump is to feed the large motors at speed. The average hp would still be whatever the engine makes, but the large tank and motors would allow bursts of high hp/tq.
 * From gtadmin: Note that you will achieve a 0-60 time of 29.92s (at 100% efficiency!) provided that you can achieve the 326ft-lbs torque at the speeds required. Unfortunately, that is not the case with your setup. The second file (hydraulic_calcs_4_real.pdf) is based on your current parameters, ignoring any accumulator. If you have dropped the idea of running the motors in series (recommended) then forget this file. I'll assume parallel operation from hereon...All is not lost if you have an accumulator that can supply 10.5 gallons in that 30 secs while still maintaining a minimum pressure of 3000psi! That is, after using 10.5 gallons from the accumulator, it still needs to have a volume of fluid at 3000psi. It appears that you need a really big accumulator - about 32 gallons pumped to 4500psi...But only 141.5 ft-lbs of torque is required to maintain a speed of 70 mph...You still have to find a way to supply 34.5gpm to maintain 70mph...To meet the accumulator requirement (which you cannot do on brake regen alone), you probably need to obtain a different pump capable of 4500psi @ 9- 10gpm, which requires the ICE to be about 38hp. However, what to do about 4WD?
 * From gtadmin: When you changeover from parallel to series, there will be 'hammer'. An accumulator will help with this problem.
 * This experiment is an exercise in efficiency. I am intentionally pushing the hp to the minimum as this will give me the best possible mpg number. As I increase engine, motor, and pump size, the mpg will only go down. In fact if I designed this thing to only go 55mph I could get radically better mpg, so I consider it a luxury to design it for 70...I fully plan to explore aero mods (belly pan, mirror delete, air intake size reduction, rear wheel skirts etc.) which have been proven by others to radically reduce hp needed at speed. So if my system only goes 65, then I will just keep modding it till it goes 70...I also plan on making a "ram" (funnel shaped) intake in the front (highest pressure area) of the car. This will act as a supercharger and gain 1-3? hp at high speeds. Of course, I could put a Real turbo on it... fuel injection, higher compression etc... the airplane folks mod this engine to put out about 26-29 hp. Supposedly it puts out 22 stock...Yet another technique to increase performance is to strip the car. Much weight can be eliminated from a production car. The things I might add are a 6-point cage, strut braces and racing seats/harnesses (this car has no airbags). Decreasing tire width and diameter will also increase ground hp...I very much agree that there is a serious hammer issue when shifting from parallel to series and back. The engine and pump will suddenly increase or decrease by a factor of 2 (as will fluid speed). This will be a stressfull shift. I agree that a small accumulator would help here, though I am not sure yet that I will use one. I plan on trying to find a "soft shift" valve that will soften the blow. I also plan to use a "clutch" valve (also soft shift) that is really a bypass around the pump. This will be hooked to my clutch pedal. So when I go to shift I will push in the clutch just like a manual transmission shift. This will "decouple" the stresses of the shift from the engine and pump. I assume I will quickly learn the correct amount to adjust the swashplate during the shift for a smooth engagement when I let up the "clutch". If the swash is hand-controlled I can create indexed positions which are best for the shift (much like the shift positions in a modern automatic (or a bicycle).
 * My car has a weight problem considering the low hp and tq values of my "unboosted" system. While I should be able to achieve my target speed of 70mph under most conditions (fairly flat and not too windy), the acceleration looks somewhat dismal on paper. This will be taken care of(hopefully) with a "boost system", a separate system designed to store and release energy. I guess it's time to start designing it...
 * As for why to use differentials: 1. I got a great deal on 2 motors that were sized correctly to run through a diff, but were the wrong size to run straight to the wheels using my pump. The diff multiplies the torque by 3.545, which is nice. If I ran to the wheels I only need 1000 rpm, but to get the same torque I would have to run all the motors in series if I used the same pump. 2. Imagine the complications with each wheel producing slightly different torque. Say the left front wheel is pushing most and alters your steering alignment... as pressure rippled through the system it could make it squirly. It would likely need some sort of balancing adjustment. This way it is balanced left to right, with the only discrepency front to back... no different than most AWD setups. 3. This setup is more easily upgradable. If I want larger motors I only need 2, not 4. Smaller motors aren't really that much cheaper, so buying 4 increases the cost of the motors, plus all the extra hose...
 * It weighs heavy on me, but I must admit my doubts about the 19.2cc gear motors having enough starting torque.  While the max torque would be ~300ftlbs, the starting torque might only be around 250 or so. According to my rolling resistance calculator, this means I could have trouble starting out in mud, snow, sand, on a hilly driveway, etc.   Sollution? bigger motors -right? except my pump can't feed bigger motors and reach my target top speed   sure, there is the "auxiliary" drive system, but I want that to be an add-on. I want the basic system to be functional by itself. So.. I purchased an open circuit variable pump of 2.75cuin (45cc) that puts out 32gpm compared to 18gpm from the other pump. This would allow me to upsize my motors to around 2cuin (33cc) and put out 450-500ftlbs of starting torque. That should allow me to start out on grass, mud, etc.
 * Over the summer I have mulled my design and decided to change a few things in the short term. The most significant is I am going to use a variable piston motor to drive the rear diff. No front motors for now as space was somewhat tight and the engineering more complex. This will simplify my hydraulic system as well. It will start out as a simple open circuit using both the variable pump and variable motor. The motor goes from 31-58cc or 1.89-3.54cuin, so it will produce ~140ftlbs@3000psi (compared to only 92ftlbs on the old design) which is about 480ftlbs at the rear wheels. The rpm limit is 4000rpm continuous, so it should be good for at least 85 mph. I expect much better starting torque and initial acceleration since the motor can produce about 50% more torque. Of course, accel at higher speeds will be limited by the engine hp.
 * more is to be gained through energy storage, as it is what allows the differentiation between "continuous hp" and "intermittent hp". The larger the storage, the greater the difference between the two. This allows a small engine but big acceleration...I believe the secret to fantastic mileage is a very small engine coupled to large energy storage, be it electric, hydraulic, or mechanical (or a combination). If you graph the energy used in driving, it spikes every time you accelerate, then drops WAY down when you reach cruising speed.
 * The calculated 0-60 time for my 22hp car is around 30sec (2000lbs). Very sad and slow by any standard (even French cars). BUT- if I can add a 3000psi 5gal accumulator, I can (in theory) get that time down to about 15 seconds.  -about the same as a Subaru, Tercel or Triumph. Not fast, but perfectly driveable. Just for comparison, pickup tow tests regularly take over 20 secs for 0-60 and are barely reaching 60mph in the 1/4 mile under max acceleration. Real world tow vehicles are likely to use at LEAST 30 seconds for 0-60. People tow trailers every day and they are not deemed "dangerously slow". A little extra care is required when pulling onto a busy highway, but other than that, they get along just fine. -So I think the initial design will be roadworthy.
 * I should have listened to everyone and waited for a good deal on a 5gal PISTON accumulator to mount horizontally where the driveshaft used to be. Instead, I purchased a Parker/greer 3000psi 5gal bladder accumulator which I now regret. I was planning to mount it more or less vertically on the firewall, but now that I have the car apart, I find the Talon, bless it's soul, has a large structural crossmember on the lower part of the firewall, so that plan is nil. It would work well on a larger vehicle though. The Talon is vertically restricted.
 * Electric has very poor regen. (batts absorb energy slow). So electric-powered hydraulic pump and hydraulic regen (accumulator) is best system on paper.
 * With all the recent Toyota recalls, I wanted to post about the dangers of interconnecting braking and regen systems. It is not uncommon that folks suggest that I put the regen on the brake pedal. But I will not. Early on, I decided that the brakes would remain only the brakes. Regen must be a different system. If a giant engineering firm like Toyota cannot successfully combine regen and braking into one, then it is inconceivable that a shade-tree builder such as myself would attempt it. I just want to remind everyone out there that safety is #1. Who cares if you get 100mpg or build a gnarly reverse trike if you slam into the back of a semi because of control failure. In my design, pressing on the brakes will trip a switch which will cut the power to the main control valve. Without power, the spring-centered valve will return to nuetral in a maximum of ~.085 seconds (If I add the centering time of the pilot valve to the centering time of the manifold). In actuallity, these actions would be somewhat simultaneous so the time would be less. So even if I am at max regen (which would cause me to stop and accelerate backwards, tapping the brakes would immediately shift the system into "neutral" mode to prevent this unwanted effect. In my swather, standing on the brakes has almost no effect if the hydrostat is not centered (they are used for turning, not slowing). Because of the power of hydrostatic drives, I believe it is imperative that use of the brakes cause centering of the swash. It is very unintuative to pull back on the lever if you are not used to it. Folks who contemplate building their own drive systems must consider safety heavily. I wonder if insurance would even cover an accident caused by a poorly designed "home-brew" drive system. As we work to develop the potential for hydraulic drive systems, we must make sure that we do not create bad press for these sytems.
 * To calculate your wheel rpm from 0 to 60 mph in 5 sec take 60-0/2 to get an average speed of 30 mph, then use the formula rpm= (336 x MPH)/(Dia of wheel), then take the rpm/60 to get revoulutions per second, then take revolutions per second x 5 seconds to get your wheel rpm to go 0 to 60 in 5 sec. Lets say you have 24" diameter wheels then rpm= (336 x 30)/(24)= 420 rpm, 420/60 seconds= 7 revs per sec, then 7 x 5= 35 wheel rpm to go from 0 to 60 in 5 sec.
 * get a piston accumulator. They do not have the problem of the diaphram getting sucked into the gas port. The piston slides all the way to the end, compressing the gas, so there is no wasted space.
 * I am trying to get the numbers to work on a 10sec 1/4 mile car. No engine or pump. Precharge in the pit and then the run is just dumping the accumulator through the motor into a holding tank. I can ALMOST make the numbers work with 30 gallons at 5000psi. At 6000psi it works a lot better, and at 7000psi it is easy. Going to 40 gal also makes the calc come out better. That is 240 gpm! If divided into 4 motors that would be 60gpm each.
 * I should note that I finally get how much better a manifold is than separate valves. It would be WAY simpler to put in one block than try to mount 4 or 5 valves (not to mention filter and cooling) like I am currently doing.
 * Ok, why use a pump which can stall my engine? (at max flow and max pressure) The hydraulic motor needs to be a certain displacement in order to move the load. To get the car up to 75mph the motor requires a certain fluid flow from the pump. This is no problem as long as I don't try to produce both high flow and high pressure at the same time.
 * Hp does not move vehicles. Torque does. Once the torque is sufficient to move, then increasing hp increases the rate of acceleration. This is VERY important in hydraulics because pressure and volume determine starting torque. If you are trying to go up a steep driveway and your starting torque is too low, you will not move, even if the car drives fine on flat ground.
 * last post at 3-15-2011

