Kinetic Energy Recovery: Ideas for Storage & Use

Hey fellow mechanics,

I've been thinking about how to store and reuse braking energy without converting it into electricity and storing it in batteries or capacitors. The background is that an Australian inventor has built a kind of hydraulic energy storage system for trucks, which builds up pressure during braking and then releases this pressure to accelerate the truck.
Here's what came to mind:
A diesel engine has a relatively high compression ratio, which makes it suitable for use as a compressor. If one needs to (slightly) brake (during normal city driving or on the highway), the engine could be run as a compressor, and the air drawn in could be compressed and stored in a tank.
To achieve this, each cylinder would only need an additional (electrically controlled) valve that opens during the compression stroke. The piston then forces the air into the pressure accumulator. Diesel fuel is not injected anyway due to the throttling.
A TDI (Turbocharger with Direct Injection) should be able to build up a pressure of approximately 18-19 bar within a short time, depending on the size of the turbocharger. Since the compression pressure is missing in the 'working cycle without injection,' the braking effect of the engine should be relatively good.
Now the question is, what can be done with the compressed air.
One idea would be to reuse them in the acceleration phase that follows the braking phase.
Here's a possible translation:

'One possibility would be to reintroduce the compressed air into the intake manifold via an additional valve. Additionally, a type of throttle valve or check valve would be needed to prevent the compressed air from escaping towards the turbocharger. The amount of compressed air introduced would be regulated by a pressure sensor to a reasonable maximum. The advantage would be that even at idle, a maximum boost pressure would be available, giving the engine a significant power boost even at low RPMs.'
2. Alternatively, one could allow air to flow back into the cylinders through the aforementioned valve during the working stroke, thereby replacing the combustion pressure during startup. It is clear that no fuel should be injected in this case. The engine would then operate similarly to a steam engine. This should be sufficient for starting in urban traffic. Large ship diesel engines are started in this way. This should also work with smaller engines.

The pressure accumulator could be designed as an isolated tank. This would have the advantage that the thermal energy generated by the compression would be retained. An alternative would be a tank consisting of several tubes arranged beneath the vehicle (for space reasons).
Theoretically, it should work like this. In practice, however, it would be quite complex, as it would require the design of both an electrical control system and mechanical components.

What do you think about it?

Greetings.

Eike.

In principle, it sounds good. However, the effort involved is enormous. This also increases the risk of errors. And the cost savings will probably not justify the effort.

Yes, yes.

It sounds very good, at least in theory.

As you describe it, it certainly could work. However, due to very sophisticated control systems and significantly higher hardware requirements, it's unlikely to be produced in large quantities, at least not in the next few years.

Respect, a good and useful idea.

Best regards, Georg.

Hello,

"Good idea. The disadvantage is the low efficiency of pneumatically powered machines. I think you'll be hard-pressed to achieve more than 30% efficiency of the braking energy due to compression, temperature losses in the storage, and losses during the working cycle. Furthermore, the storage systems are heavy. Compared to existing hybrid vehicles (Prius, Insight), the comparison in terms of weight, complexity, and cost might not be so bad (the overall efficiency of the electric drive, from braking energy to propulsion, probably won't exceed 60% either). Especially when you consider the complexity of the Prius, with its two electric motors, special transmission, and NiMH batteries."
Compressed air systems were used in various applications, including mining locomotives (e.g., manufactured by Ex Schutz), torpedoes (where protection from explosion was less critical), and underwater scooters. Larger diesel engines have long been started using compressed air, as suggested.

Is anyone aware of whether a system like this has ever been tested by industry before, specifically by someone named Eike?

Best regards,
Christian.

http://www.umweltfibel.de/lexikon/d/lex_d_druckluftauto.htm

http://www.aircar-luftauto.de
http://www.aircars.de


please note the operating conditions


CU Gremlin.

There was a Golf 3 model called "Ecomatic" once. Didn't it have some kind of (mechanical, flywheel-based) energy recovery system?

Hi!

When using a diesel engine in a car as a compressor, the compressing piston works against the pressure in the pressure accumulator. This counter-pressure determines the braking torque.

For moderate braking performance and to prevent wheel lock-up, a relatively low pressure of approximately 10 bar is required. (This is due to the relatively large piston area of the diesel engine).

With such low storage pressure, the storage tank must have a very large volume in order to store usable amounts of energy. Furthermore, in a rigid storage system, the pressure is highly dependent on the fill level, meaning that the braking effect would not be constant and would increase with a higher fill level.

A solution can be found in bladder-type pressure vessels, which, due to their bladder design, maintain a nearly constant pressure regardless of the fill level.

The question remains: where can you possibly fit several cubic meters of compressed air in a car? I can imagine a rubber bladder on the roof of the car that would inflate into a balloon with a diameter of 1.50 to 2 meters.

Best regards,