Regenerative compression braking

Honest John Back Room » Motoring discussion
What is life like with your car? Let us know and win £500 in John Lewis vouchers | No thanks
View Threaded
Fri 27 Jul 2018 18:37
Regenerative compression braking - mss1tw

Why don't manufacturers fit an air tank (Like those found on HGVs for the brakes - standard industry tech) and use the 'wasted' compression strokes as a brake to fill the tank, and use the compressed air like a small battery for stop-start traffic?

The engine could still run and power A.C. etc etc that way, with nothing but air out the exhaust.

Maybe there are catalyst concerns but seem ok with standard stop-start systems

Reply | Report as offensive | Link
Fri 27 Jul 2018 19:08
Regenerative compression braking - Andrew-T

Why don't manufacturers fit an air tank (Like those found on HGVs for the brakes - standard industry tech)

Possibly because it would need some space somewhere?

Reply | Report as offensive | Link
Fri 27 Jul 2018 19:23
Regenerative compression braking - badbusdriver

Why don't manufacturers fit an air tank (Like those found on HGVs for the brakes - standard industry tech) and use the 'wasted' compression strokes as a brake to fill the tank, and use the compressed air like a small battery for stop-start traffic?

The engine could still run and power A.C. etc etc that way, with nothing but air out the exhaust.

Maybe there are catalyst concerns but seem ok with standard stop-start systems

They don't really need to do they?. A truck, even a small one, needs considerably more braking force than a car, especially on a long downhill section. Granted the transmission brake would also be used to control the speed in that case, but the forces truck brakes must deal with are immense compared to a car, even a big porky SUV. Also, a 3 axle tractor unit with a 3 axle trailer has 12 sets of brakes, can you imagine trying to operate that lot on battery power?.

With the rise of hybrids and pure electric cars, they already harvest the braking power to recharge the battery and quite a few more expensive cars these days are moving over to 48 volt systems so there will be no shortage of spare electrical power.

I think some of the bigger American pickups do use airbrakes, but some of them can have a 'train weight' of close to a truck anyway.

Reply | Report as offensive | Link
Fri 27 Jul 2018 20:32
Regenerative compression braking - bathtub tom

I suggest the OP googles jake brakes.

Reply | Report as offensive | Link
Sat 28 Jul 2018 08:12
Regenerative compression braking - jc2

Not only jake brakes-exhaust brakes but also transmission retarders.

Reply | Report as offensive | Link
Fri 27 Jul 2018 20:37
Regenerative compression braking - mss1tw

I think you're focussing on my brakes comment too much - I was thinking of using standard parts to use the engine as a pure air pump and storing a small amount of energy that way.

Existing tech, no toxic batteries or additional motors, just a small air tank.

Reply | Report as offensive | Link
Fri 27 Jul 2018 21:19
Regenerative compression braking - badbusdriver

How is a small compressed air tank going to start your car when you get back from a couple of weeks in Spain?.

Reply | Report as offensive | Link
Fri 27 Jul 2018 21:45
Regenerative compression braking - Energyman
Wasn't the OP suggesting the air tank be used to power the Eco/stop start system instead of battery and starter motor.
I assume it would need a seperate starter so could be complicated.
Don't some use a capacitor instead of the battery to operate the starter motor? Or have I made that up?
Reply | Report as offensive | Link
Sat 28 Jul 2018 00:09
Regenerative compression braking - FP

"Don't some use a capacitor instead of the battery to operate the starter motor? Or have I made that up?"

My Mazda CX-5 uses a capacitor in its stop-start system.

"In 2011, Mazda announced i-ELOOP, a system which uses a variable-voltage alternator to convert kinetic energy to electric power during deceleration. The energy stored in a double-layer capacitor, is used to supply power needed by vehicle electrical systems. When used in conjunction with Mazda’s start-stop system, i-Stop, the company claims fuel savings of up to 10%." (Wikipedia)

Reply | Report as offensive | Link
Sat 28 Jul 2018 08:55
Regenerative compression braking - badbusdriver
Wasn't the OP suggesting the air tank be used to power the Eco/stop start system instead of battery and starter motor. I assume it would need a seperate starter so could be complicated. Don't some use a capacitor instead of the battery to operate the starter motor? Or have I made that up?

This is what the OP says at the end of their last post,

"Existing tech, no toxic batteries or additional motors, just a small air tank."

I could be wrong, but to me this suggests the OP is not thinking of getting rid of the battery completely. While i am no engineer, i just don't see how that could be made to work.

Reply | Report as offensive | Link
Sat 28 Jul 2018 22:47
Regenerative compression braking - badbusdriver
Wasn't the OP suggesting the air tank be used to power the Eco/stop start system instead of battery and starter motor. I assume it would need a seperate starter so could be complicated. Don't some use a capacitor instead of the battery to operate the starter motor? Or have I made that up?

This is what the OP says at the end of their last post,

"Existing tech, no toxic batteries or additional motors, just a small air tank."

I could be wrong, but to me this suggests the OP is not thinking of getting rid of the battery completely. While i am no engineer, i just don't see how that could be made to work.

Just realised that i typed "the OP is not thinking of getting rid of the battery completely." Instead of the OP is thinking of getting rid of the battery completely, which is what i meant!.

Reply | Report as offensive | Link
Sat 28 Jul 2018 09:20
Regenerative compression braking - mss1tw
Wasn't the OP suggesting the air tank be used to power the Eco/stop start system instead of battery and starter motor. I assume it would need a seperate starter so could be complicated. Don't some use a capacitor instead of the battery to operate the starter motor? Or have I made that up?

The air tank supply would be run back through the engine as required for slow speed town work and idling. So you would be 'running' the engine on the stored air to keep accessories working, and for slow town work, with the fuel injection kicking in and the stored air being cut off and returning to normal combustion as required.

Then on the over-run the opposite would apply - the fuel injection already cuts off as standard, so it would just mean routing the 'exhaust' into the air tank while no throttle is applied

Reply | Report as offensive | Link
Sat 28 Jul 2018 12:31
Regenerative compression braking - corax

It's been experimented with, but if you google articles on compressed air energy storage, it's more suited to large scale power production, or transportation systems like mine locomotives.

Maybe the complexity of producing a system that is only in use part of the time outweighs any efficiency gains. You would need extra space for the tank and other ancillaries like heat exchangers.

en.wikipedia.org/wiki/Compressed-air_vehicle

Reply | Report as offensive | Link
Sat 28 Jul 2018 19:07
Regenerative compression braking - mss1tw

It's been experimented with, but if you google articles on compressed air energy storage, it's more suited to large scale power production, or transportation systems like mine locomotives.

Maybe the complexity of producing a system that is only in use part of the time outweighs any efficiency gains. You would need extra space for the tank and other ancillaries like heat exchangers.

en.wikipedia.org/wiki/Compressed-air_vehicle

Ahhh! Oh well, back to work Monday then

(Cheers for your infomative reply though)

Reply | Report as offensive | Link
Sat 28 Jul 2018 22:08
Regenerative compression braking - KJP 123

I think that the answer is that it would cost manufacturers - a cost passed onto consumers - for little consumer benefit.

I had not heard that 48v electrics were being considered; twice that of HGVs.

I recall from 1970s cars that there were brake servo reservoirs. These seem to have disappeared or am I just wrong?

Edited by KJP 123 on 28/07/2018 at 22:09

Reply | Report as offensive | Link
Sat 28 Jul 2018 22:51
Regenerative compression braking - badbusdriver

I think that the answer is that it would cost manufacturers - a cost passed onto consumers - for little consumer benefit.

I had not heard that 48v electrics were being considered; twice that of HGVs.

I recall from 1970s cars that there were brake servo reservoirs. These seem to have disappeared or am I just wrong?

48v electrics are not 'being considered', they are already in use. The Bentley Bentayga, Audi A8 and Q7 have them. And i'm pretty sure others do too.

Reply | Report as offensive | Link
Sat 28 Jul 2018 23:53
Regenerative compression braking - bathtub tom

I recall from 1970s cars that there were brake servo reservoirs. These seem to have disappeared or am I just wrong?

Turn off your engine and press the brake pedal, you'll find you have a few assisted stops before you lose servo assistance.

Reply | Report as offensive | Link
Mon 30 Jul 2018 15:02
Regenerative compression braking - Stackman II

I seem to remember a few years ago that Peugeot were investigsting the use of compressed air as a form of hybridisation of some of its cars.

Reply | Report as offensive | Link
Wed 8 Aug 2018 14:40
Regenerative compression braking - Glaikit Wee Scunner {P}

A real safety problem is that compressed air cum fuel/engine oil is likely to spontaneously ignite. There would need to be a sophisticated system to collect the fuel/oil before it went into the tank. Air compressor explosions are a real problem.

And yes, vacuum reservoirs are still fitted to car braking systems. I couldn't point to mine but I can see the vacuum pipe. I think it is rather hidden behind the brake fluid reservoir.

Edited by Glaikit Wee Scunner {P} on 08/08/2018 at 14:50

Reply | Report as offensive | Link
Thu 9 Aug 2018 14:27
Regenerative compression braking - davecooper

Some time ago I wondered why you couldn't fit a hydraulic pump to each wheel instead of the friction braking system and use the action of pumping oil into an accumulator to slow the car down instead of conventional brakes. The pressurised oil could then be fed back to the pump which would now act as a motor and provide initial take off power. I could see some problems with this such as being able to provide variable braking force. However, I then discovered that such a system is alread used in heavy plant machinery where it is driven by hydraulic motors. The motors become pumps under braking.

Correct me if I am wrong but I don't believe there is a car regenerative braking system that harnesses and stores all of the braking energy at the wheels. A disc brake is always used as well which wastes energy as heat.

Reply | Report as offensive | Link
Thu 9 Aug 2018 20:58
Regenerative compression braking - bathtub tom

I then discovered that such a system is alread used in heavy plant machinery where it is driven by hydraulic motors. The motors become pumps under braking.

Hydraulic fluid is non-compressible and therefore there is no way of of storing compressed hydraulic fluid. Hydraulic fluid can be used for transferring energy (as in hydraulic brakes or transmission).

Reply | Report as offensive | Link
Thu 9 Aug 2018 22:00
Regenerative compression braking - SteveLee

Some time ago I wondered why you couldn't fit a hydraulic pump to each wheel instead of the friction braking system and use the action of pumping oil into an accumulator to slow the car down instead of conventional brakes. The pressurised oil could then be fed back to the pump which would now act as a motor and provide initial take off power. I could see some problems with this such as being able to provide variable braking force. However, I then discovered that such a system is alread used in heavy plant machinery where it is driven by hydraulic motors. The motors become pumps under braking

Great idea, except compressing liquids is,er, kind of against the laws of physics, unless all you're compressing is the air space in the accumulator which wouldn't be much.

Reply | Report as offensive | Link
Thu 9 Aug 2018 23:16
Regenerative compression braking - davecooper

You use an air or nitrogen loaded accumulator. We use them extensively on Warships to provide a back up in case of hydraulic plant failure. A bladder in the accumulator is pressurised with high pressure gas. The hydraulic fluid on the other side of the bladder compresses the bladder so providing a reserve of oil at high pressure if required. However, due to the pressures involved, the accumulators are by necessity very heavy.

Reply | Report as offensive | Link