124 replies to this topic

[just me again]

Last week i was towing a caravan, and me being used to drive a Scania every fourthnight, my left foot seeked after the exhaust brake button when going downhill. So have there been any cars with exhaust brakes, jacobs brakes or retarders? and if not, why?.
If a truckdriver can mannage an extra button, then it should be possible for "normal human beings" also.

Bjørn

[cheapracer]

Last week i was towing a caravan, and me being used to drive a Scania every fourthnight, my left foot seeked after the exhaust brake button when going downhill. So have there been any cars with exhaust brakes, jacobs brakes or retarders? and if not, why?.
If a truckdriver can mannage an extra button, then it should be possible for "normal human beings" also.

Bjørn

On petrol engines I believe it leads to shortened exhaust valve life and the lower compression won't give the same effect as a diesel.

[MatsNorway]

Just a question

On petrol engines I believe it leads to shortened exhaust valve life and the higher? compression won't give the same effect as it would do on a diesel.

[Tony Matthews]

On petrol engines I believe it leads to shortened exhaust valve life and the lower compression won't give the same effect as a diesel.

[MatsNorway]

lol sorry i meant:
The rise in compression due to blocking the exhaust gases causes the valve damage?

[gruntguru]

I can't see a problem with exhaust valve life provided fuel is cut whenever exhaust brake is activated. Low compression ratio shouldn't be a big problem. The system relies on increasing the pumping work during the exhaust stroke and in same cases (using additional exhaust valve actuation) the compression stroke as well. The throttle would probably need to be opened as well (not applicable on Diesels).

[just me again]

I found this

http://www.jakebrake.../ld-exhaust.php

You can get an exhaustbrake for yours Cummins engined Dodge Ram, While not being a small car, it´s still smaller than a full size truck.

I was wondering with the Toyota Prius, can it brake with it´s electro motor, and does it have a resistor to burn the energy in if the batterys is fully charged.

And also, have there ever been any petrol engined trucks or cars with exhaust brakes. I think, like gruntguru wrote, that it can be made to work on petrol engines.

Bjørn

[Catalina Park]

There have been petrol engined vehicles with exhaust brakes in the past, although they were not that common they were around in the 50s and 60's.

[gruntguru]

I was wondering with the Toyota Prius, can it brake with it´s electro motor, and does it have a resistor to burn the energy in if the batterys is fully charged.

Yes and No. Not sure what the Prius algorithm does when the battery nears full charge. It would make sense to run "electric only" for a while to make sure there is storage capacity available for regenerative braking. It would be a shame to arrive at a long downhill run with the battery full charged.

[J. Edlund]

Why would be the big question?

For starters, an exhaust brake and a jacobs brake or jakes brake are two different things. An exhaust brake is basically a butterfly valve in the exhaust system which increases the exhaust back pressure when closed. As the gasoline engine uses a throttle before the engine a second valve in the exhaust system wouldn't be of much use.

The jakes brake is a compression release brake; it releases the pressure created during compression which means the engine will operate as a compressor, which consume power. This is however quite noisy, and nothing I would recommend for a road car.

Finally we have the question why? For a heavy truck an exhaust brake can be useful during long downhill roads where the heavy cargo the truck is carrying would increase the speed of the truck unless the brakes are used. To use the brakes for such a long time would however overheat the brakes. So for a heavy truck it's quite a useful function. For a passenger car, there isn't really any need though as the normal engine brake function will provide all the braking needed without additional systems or buttons.

As for the Toyota Prius, it's battery is never fully charged. I can't remember the exact state of charge used, but a hybrid car typically keeps the battery in a 30-80% state of charge or similar, so there is always reserve capacity. If the battery charge is getting low, the system will charge the battery and of the battery charge is getting high it will discharge the battery.

[gruntguru]

An exhaust brake is basically a butterfly valve in the exhaust system which increases the exhaust back pressure when closed. As the gasoline engine uses a throttle before the engine a second valve in the exhaust system wouldn't be of much use.

If the intake throttle was opened wide (easy with DBW) and fuel shut off, an exhaust brake would provide greater braking torque than intake throttling.

[desmo]

If the intake throttle was opened wide (easy with DBW) and fuel shut off, an exhaust brake would provide greater braking torque than intake throttling.

OK I'll bite, why?

[Greg Locock]

greater mass of air being worked on?

[gruntguru]

greater mass of air being worked on?

That's probably right. I approached it from another direction - pressure difference across the engine (compressor) is a maximum 14.7 psi with intake throttling, roughly 10 times that (with say 10:1 compression ratio) using exhaust throttling.

Edited by gruntguru, 01 October 2010 - 07:14.

[desmo]

Good stuff, I was viewing it from the simplistic point of view of the entire four stroke cycle where the massflows in and out of the cylinder will obviously be the same.

[cheapracer]

I can't see a problem with exhaust valve life provided fuel is cut whenever exhaust brake is activated.

Low compression ratio shouldn't be a big problem.

Yeah showing my age - carbies

You know very well that compression ratio is an issue for it, thats the force that brakes the engine and 18:1 will always be far superior than 9:1.

[gruntguru]

You know very well that compression ratio is an issue for it, thats the force that brakes the engine and 18:1 will always be far superior than 9:1.

On one hand, the compression work is 54% higher for the 18:1 case so the ideal "Jake Brake" which uses compression work (then releases the gas with specially timed exhaust opening) will be up to 54% better. On the other hand, with a "throttled-exhaust" braking system, the entire exhaust stroke will be performed at whatever back-pressure the system is designed for, so the work (braking) will be the same for both. In theory the high compression engine could achieve a higher BP but I think 840 psi BP would be problematic. (even 9:1 compression = 319 psi)

(All figures assume ideal gas and isentropic compression. Handy calculator )

[just me again]

I was looking on the data for the Toyota Prius and Honda Insight and none of them can pull a trailer, WHY! they should actually be prefereble to a "normal" car with theres braking capabilities and the economy of the hybrid systems.
My Grande Punto can pull 1000kg with only 77HP. so a Prius or a Insight have power to pull much more.
Maybe an exhaust brake or retarder is overkill for a car, but if you are using it too pull heavy trailers it will add comfort and security to the driving .

Bjørn

[MatsNorway]

I was looking on the data for the Toyota Prius and Honda Insight and none of them can pull a trailer, WHY! they should actually be prefereble to a "normal" car with theres braking capabilities and the economy of the hybrid systems.
My Grande Punto can pull 1000kg with only 77HP. so a Prius or a Insight have power to pull much more.
Maybe an exhaust brake or retarder is overkill for a car, but if you are using it too pull heavy trailers it will add comfort and security to the driving .

Bjørn

The chassie might not take it on the prius.

[Greg Locock]

The chassie might not take it on the prius.

That's it. To meet trailer durability requirements would probably add a few kgs of steel to the car, which would serve no good purpose for 99% of their customers, and would affect their headline fuel and emissions numbers.

[Zoe]

I was driving the Grossglockner mountain pass road just yesterday, and the engine brake of my car was not adequate for going 12% downhill simply with engine braking without accelerating up to unhealthy speeds without using the brakes. I did the same tour previously with a different car, and it was possible to drive downhill most of the time without using the brakes (only before switchbacks).

One of the cars is a turbo, the other an n/a, both are gasoline engines. One turbo car has 140 hp for 1500kg empty weight, the n/a has 170 hp for 1200 kg empty weight.

Now, which of the cars would have been the turbo one?

Zoe

[J. Edlund]

That's probably right. I approached it from another direction - pressure difference across the engine (compressor) is a maximum 14.7 psi with intake throttling, roughly 10 times that (with say 10:1 compression ratio) using exhaust throttling.

Turbocharger manufacturers typically specify around 450 kPa max for exhaust brakes, so it would be more like 100 kPa vs. 450 kPa för exhaust brake applications. What compression ratio is used would not have a significant effect as the 'braking' is performed during the intake or exhaust stroke.

It would however be a better solution to increase the braking with for instance an electric generator.

Edited by J. Edlund, 04 October 2010 - 15:35.

[Tony Matthews]

I was driving the Grossglockner mountain pass road just yesterday, and the engine brake of my car was not adequate for going 12% downhill simply with engine braking without accelerating up to unhealthy speeds...

Zoe

I drove down there in a Mk9 Jag, towing a FFord on a trailer - one front brake failed, the Jag was an auto... Fortunately there was a goat track running up from the next hairpin, and we bounced to a stop thirty metres up the track, and decided what to do next...

[Zoe]

Beats my why one would consider doing the Grossglockner with a trailer, and then with an Auto as well - but I've seen Dutch people with their RV trailers getting stuck in places I wouldn't have dared to go with a regular car, so I guess I shouldn't be too surprised (I guess your adventure must have happened on the South side, since on the North side emergency run-offs are pretty much non-existent).

Back to the topic of engine braking; someone once told me that it is not healthy for a gasoline engine to be used as car brake on long downhill sections (i.e. revving at 3-4000-ish rpms with no throttle). Is that correct or an urban legend? (my cars have so far all survived my trips into the mountains).

Zoe

[Tony Matthews]

OT I know, but North side, we took a wrong turning going from Innsbruck to Salzburg and nearly got to Graz before turning North again... Happy days!

[mariner]

THe Jacobs website also contains this interesting page which implies that Jacobs is developing something roughly similar to FIAT multijet

http://www.jakebrake...y/vva_works.php

The PDF download gives more info. They are claiming that the Jacobs VVA sysem can help to deliver HCCI engines which iis a big step if true.

[gruntguru]

One of the cars is a turbo, the other an n/a, both are gasoline engines. One turbo car has 140 hp for 1500kg empty weight, the n/a has 170 hp for 1200 kg empty weight.
Now, which of the cars would have been the turbo one?

Logic says the N/A car would have the bigger displacement, and being a lighter car (you didn't give the "loaded" weight) should have had more downhill retardation available.

Now tell us why logic is wrong!

[Zoe]

Nah, I didn't question logic; I was just asking to confirm my suspicion (and first-hand experience).

Sometimes you think of something, which then turns out to be something else....

But is it because compression ratios are different that a turbo engine has less retardation force than an n/a engine?

Zoe

[gruntguru]

But is it because compression ratios are different that a turbo engine has less retardation force than an n/a engine?

I think the higher compression will make a small difference only.

[MatsNorway]

Back to the topic of engine braking; someone once told me that it is not healthy for a gasoline engine to be used as car brake on long downhill sections (i.e. revving at 3-4000-ish rpms with no throttle). Is that correct or an urban legend? (my cars have so far all survived my trips into the mountains).

Its about the forces the piston is designed to take and their lack of it when engine somehow revs up off throttle. you get that punch out in the air. (try it as hard as you can) Turbo cars are more gentle that way as the work under the fourth stroke as well.

++ lots of details.

[fredeuce]

I was driving the Grossglockner mountain pass road just yesterday, and the engine brake of my car was not adequate for going 12% downhill simply with engine braking without accelerating up to unhealthy speeds without using the brakes. I did the same tour previously with a different car, and it was possible to drive downhill most of the time without using the brakes (only before switchbacks).

One of the cars is a turbo, the other an n/a, both are gasoline engines. One turbo car has 140 hp for 1500kg empty weight, the n/a has 170 hp for 1200 kg empty weight.

Now, which of the cars would have been the turbo one?

Zoe

I would have thought you would need to take into account the gearing involved and the overall final drive ratio.

Clearly, an overdrive transmission will not afford the same levels of retardation as a direct drive , all else being equal.

[gruntguru]

I would have thought you would need to take into account the gearing involved and the overall final drive ratio.

Clearly, an overdrive transmission will not afford the same levels of retardation as a direct drive , all else being equal.

I assume Zoe changes gears to keep RPM at a sensible level during engine braking.

[Zoe]

You are both right, of course. In fact, the comparison of the two cars is something like apples and oranges. Not only are the cars different (one is a family van (inherited from my dad is my excuse) with a low-compression turbo engine and 1.5 tons empty weight, the other a sports coupe with a 10:1 n/a engine and 1.2 tons empty weight), engine displacement and gearing are different as well. The turbo-van is geared significantly shorter than the n/a car.

And yes, I do shift gears to keep rpms in a range that doesn't hurt my ears, and hopefully neither the engine.

Zoe

[Cavalier53]

You are both right, of course. In fact, the comparison of the two cars is something like apples and oranges. Not only are the cars different (one is a family van (inherited from my dad is my excuse) with a low-compression turbo engine and 1.5 tons empty weight, the other a sports coupe with a 10:1 n/a engine and 1.2 tons empty weight), engine displacement and gearing are different as well. The turbo-van is geared significantly shorter than the n/a car.

And yes, I do shift gears to keep rpms in a range that doesn't hurt my ears, and hopefully neither the engine.

Zoe

Zoe, i've also been struck by the small amount of engine braking my 1500 kg 1.6 150 HP turbo family van provides. Is it maybe that the throttle simply opens a little to prevent suction over the piston rings and valve stem seals now that the injection actually cuts off any petrol supply under such conditions?
That would also help to explain the unbelievable oil consumption (noting added in 20 k KM).

[J. Edlund]

Back to the topic of engine braking; someone once told me that it is not healthy for a gasoline engine to be used as car brake on long downhill sections (i.e. revving at 3-4000-ish rpms with no throttle). Is that correct or an urban legend? (my cars have so far all survived my trips into the mountains).

Zoe

Engine braking is only a problem for two strokes which use oil blended gasoline, for a modern four stroke having a separate lubrication system there wouldn't be any problem at all. The load put in the engine during engine braking is far lower than what the engine see during normal operation.

[Greg Locock]

Engine braking is only a problem for two strokes which use oil blended gasoline, for a modern four stroke having a separate lubrication system there wouldn't be any problem at all. The load put in the engine during engine braking is far lower than what the engine see during normal operation.

Anecdotally though high speed zero load was the classic way to fail a proto engine on the dyno.

[gruntguru]

Anecdotally though high speed zero load was the classic way to fail a proto engine on the dyno.

Interesting. What sort of failures?

[Greg Locock]

Interesting. What sort of failures?

Can't remember, I'd guess pistons.

[gruntguru]

This has always intrigued me and I have usually treated it as something of an urban myth. After all doesn't a race engine see zero throttle at peak rpm every gear change? (at least in the good old days of clutch and H pattern)

[cheapracer]

Can't remember, I'd guess pistons.

I've twice seen 202 Red Holden motors break the piston tops of on big free revs.

This has always intrigued me and I have usually treated it as something of an urban myth. After all doesn't a race engine see zero throttle at peak rpm every gear change? (at least in the good old days of clutch and H pattern)

What, flat changing?

Edited by cheapracer, 11 October 2010 - 03:06.

[gruntguru]

. . . flat changing?

Zero throttle???

[Tony Matthews]

Zero throttle???

Surely, with seamless shifting, full throttle changes are the norm? Normal, not Norman.

[gordmac]

The tensile load on the rod is highest when the piston is decelerating for tdc, on the overrun there is no compressive force from the pressure above the piston so high rpm on overrun will be worst loading case.

[cheapracer]

Surely, with seamless shifting, full throttle changes are the norm? Normal, not Norman.

Over the years I have probably fitted about 20 racing motorcycles with stop light switches on the gearshift to cut the engine as the upshift is made, you don't move the throttle at all but of course cutting the engine for a tenth of a second (whatever) unloads the box enough to change. Some bikes you can flat change such as an older Maico without doing a thing except move the shifter up.

I have never driven a Porsche Triptonic but wasn't that basically just a cut switch on the rocking gearstick knob?

Grunt, flat change means to slam it through the gears just depressing the clutch but not lifting the throttle at all and the engine revs will spike momentarily, surely you were a teenager once upon a time.

Otherwise I have no trouble changing gears without spiking motors.

[Tony Matthews]

Over the years I have probably fitted about 20 racing motorcycles with stop light switches on the gearshift to cut the engine as the upshift is made, you don't move the throttle at all but of course cutting the engine for a tenth of a second (whatever) unloads the box enough to change. Some bikes you can flat change such as an older Maico without doing a thing except move the shifter up.

Yeah, but... Seamless means seamless, no cut, the two (or three) clutches allow for full-throttle up-shifts. Unless we are talking down-shifts here - I may be confusing myself. Again.

[McGuire]

Anecdotally though high speed zero load was the classic way to fail a proto engine on the dyno.

There is a virtually limitless supply of lore on this subject... but when an engine is installed on the dyno, some unspecified mass is bolted onto the back end of the crank (as opposed to the free end, where the damper is located) via some unspecified coupler. And every once in a while, on no-load warmup at some unspecified rpm, the crankshaft snaps in half. Huh. Probably a bad crank.

[cheapracer]

Yeah, but... Seamless means seamless, no cut, the two (or three) clutches allow for full-throttle up-shifts. Unless we are talking down-shifts here - I may be confusing myself. Again.

They match the revs to the next gear so it most certainly drops.

Saw and heard my first Nissan GTR in Honk Kong last week, first saw it parked then some luck saw it take off hard from some lights 1/2 hour later and the 'seamless' shift was audibly very impressive, much more so than Ferraris that I have heard before.

It was butt ugly though.

[OfficeLinebacker]

Grunt, flat change means to slam it through the gears just depressing the clutch but not lifting the throttle at all and the engine revs will spike momentarily, surely you were a teenager once upon a time.

This reminds me of an old coworker with some muscle car in the 60s as a teen. He said he'd leave, burning rubber the whole time, and flat shift into 2nd, again leaving rubber. He said the clutch pedal return spring was faster than his leg muscles so he'd just slide his foot off the side of the pedal once it was in gear.

The big thing in his group of friends was they would actually measure the gap in the black stripe after each run and they were trying to minimize that gap. I forget what his personal best was but I think it was measured in inches.

[OfficeLinebacker]

There is a virtually limitless supply of lore on this subject... but when an engine is installed on the dyno, some unspecified mass is bolted onto the back end of the crank (as opposed to the free end, where the damper is located) via some unspecified coupler. And every once in a while, on no-load warmup at some unspecified rpm, the crankshaft snaps in half. Huh. Probably a bad crank.

Imagine that.

[Tony Matthews]

They match the revs to the next gear so it most certainly drops.

I wonder. They did that before seamless, so there can't be much benefit, except in not introducing a jerk into the system - no, not the driver. Unfortunately there is no, as far as I am aware, plan to update Peter Wright's book on this stuff. A couple or three years ago I was told that none of the gearbox people were prepared to spill the beans.

It was butt ugly though.

Some people's butt ugly is purposeful and aggressive.