Why no Hydraulic Drive Systems on roadcars?

[Conceptual]

I have spent several dozen hours trying to answer this single question, and have even looked for laws that ban its use, due to the fact that I cannot find the single point failure in the idea.

I appreciate any ideas, and I will try to cover any problems that I have already researched.

One would think that in this time of crisis, the 50hp motors that are required to drive a pump would be much more attractive than the 100hp engines that are the current trend.

Thanks in advance!

[Cyco]

Because a 50hp motor will still be a 50hp motor regardless of if it is driving via a gearbox, or a hydraulic system. People want a 100hp one so they can show off.

Hydraulic systems are great for high torque situations, but are bad at high speed applications. They can cavitate, and then they need to be bled, and the reservoirs take time to pressurise and are bulky.

[xpensive]

Correct, in popular terms mechanical power can be descibed as torque times angular velocity, while hydraulic power is pressure times volumetric flow, where the flow is the speed equivalent.

As a concequence, a higher rpm requires a higher flow which increases viscous losses through every component, pump-hoses-motor-valves. But on heavy construction machinery for example, you can very often see a diesel engine driving a hydraulic pump at low rpm, feeding individual motors on each wheel, excellent for high-torque and low speed applications.

When you think about it, an F1 engine's torque is not that impressive really, a little more than 250 Nm (340 feet-lb?), but the angular speed is awesome, why power is 700 Hp anyway.

[Conceptual]

So, a single high torque, low RPM motor powering a gearbox would not move the car appreciably?

I am thinking more like Hydrostatic, but obviously much more dynamic in control.

The place I worked for when I got hurt had this smallish dump-cart. It weighed 2000lbs, and could take a 2500lb load. It was ran by a 50hp 2 cylinder diesel motor, and would easily hit 25mph when full. I always wodered why it couldnt run my wifes work car the same, since the car is actually lighter than the full cart...

[xpensive]

The equipment you are referring to, pump and motor, are typically hydrostatic.

It is always difficult to judge how much power is needed for what, only that power (as in energy per time-unit) is the same regardless of shape or form. One thing to remember however, is that a typical Porsche only needs some 20 Hp to cruise at 55 mph, while the same car would need 160 Hp to go 110 mph.

Problem with hydraulic transmissions, again, is efficiency. See the previous discussions with riff_raff on hydraulic CVTs.

[mike]

i don know much about hydraulic drive systems but i share the same fascination with hydraulics im just wondering why there isnt a hydraulic axle in road cars
i have seen hydraulic KERS that uses a full hydraulic axle and cylinder combo
hydraulic axles should be great for packaging of the car as no axle has to run between chassis

[xpensive]

I share that fascination too, but could you please define "hydraulic axle"?

[Conceptual]

I share that fascination too, but could you please define "hydraulic axle"?

The dump cart that I alluded to earlier had this same type of axle. It is just a 2"x2" square tube, and it has a send and return connection for the hydraulic fluid. I have no idea what is inside it, but I do know that it was fitted with tyres very much like a quad runner...

[mike]

thats it without the reservoir, the reservoir is the KERS

[xpensive]

There you go mike & conceptual, the "hydraulic axle" are hoses or tubing. Nice image btw, I will save that one.
This is actually the way they are planning to arrange the transmission for windmills in Norway, with a variable displacement hydraulic pump/motor, which I mentioned on that CVT thread earlier, with the generator sitting o the ground.
Anyway, the problem is as always the viscous losses through the system, where a rotating shaft has zero, except necessary gears and joints, which can be rather small in comparison.

[Carlos]

Ciro's meta-curiosity unearthed an intriguing hydraulic device in a thread last year, which checkered (a valued contributor, the forum is certainly diminished without his investigative posts) detailed contributions. I seem to remember the inventor of the hydristor also took part. One of our best threads.

OT - Shame that in depth engineering threads and posts have got up and 'left the building' - seemed to happen when F1T got into loose concept based amusements instead of threads that included any intellectual effort and research. Things change - Guess that's a rant.

Hydristor Push To Pass
viewtopic.php?t=3577

Edit - Yes your right Xpensive, the picture aero threads inspire a lot of in depth discussions and some of the generous contributions of bar555 and certainly many other members are a resource now and in years to come. Some of our other threads are also very informative, members to of course. The threads on chassis dynamics and race circuit layout/construction, carbon fiber layup and monocoque and engine and transmissions are 'must reads'. Since I joined,I've read the entire Forum library. There's always a 'technical group' posting. I'd like to see more in-depth threads. It may be that the technology of F1 is beyond non-professionals and a stagnation in the evolution of F1. Perhaps it has reached a point of perfection I would just like more discussion, more research of technology. I know this Edit could have been sent as a PM ... but I've written it to encourage present members, past members and future members.

[xpensive]

Well Carlos, picture-aerodynamics seems to attract a lot of attention.

But seriously, as a relative newcomer here, I am surprised by the lack of interest in pure technical threads, while analyzis of testing times seems to go on forever?

[Professor]

Hydraulic systems do not work well in high speed applications, and for road cars they are quite heavy as a percentage of the total weight. They are useful on large, slow, stop and go vehicles like city busses and waste disposal trucks.

The stop-start-stop nature of their routine makes them especially suited to hydraulic systems.

[xpensive]

No wonder they call you Professor.

[Professor]

http://www.autobloggreen.com/2007/04/20 ... ic-series/

Here's a link to a UPS truck with the technology.