Originally Posted by DirtyDan (Post 1716985)

Diesels have less engine braking exactly because of their higher compression. The higher compression of a diesel provides far more force to the DOWNSTROKE of the piston than a petrol piston of the same displacement and therefore much more force turns the crankshaft and then the drive shaft etc. than is the case with a petrol engine. What a jake break does is let the air out of the balloon, let the air out of the DOWNSTROKE by venting it out the exhaust valves. This takes force away from the crankshaft and hence the rest of the vehicle. It is the DOWNSTROKE that is the real difference maker in engine braking between diesel and petrol along with the fact that the intake is open on the diesel on the upstroke and closed on the petrol.

I think it is the gearing that makes off-roaders think their diesels brake better than petrols.

Jake brakes were invented to save brakes on heavy vehicles. They might be useful in slowing the free fall of rogue planetoids like the Tata Safari.

Originally Posted by star_aqua;1717207[B

][/b]DD, lets go according to your explanation. if you consider a petrol engine and a diesel engine of same displacement, diesel has higher compression then petrol. now both work on same principle in terms of strokes.

>> Yes.

among four strokes one stroke needs compression. during engine braking, the engine is a dead engine and you are driving it through external force(wheels here). now how much energy do u need to over come the compression stroke for diesel and same stroke for a petrol. do you think here petrol engine need more energy? or diesel need less energy to compress?

>> Diesel engine needs more energy to compress and once the piston is over tdc the compressed air acts as a spring returning the energy. So most of the energy lost in compressing the air is returned.

>> Take a plastic syringe close its mouth with your finger compress the piston it will bounce back. same principle.

>> Petrol needs very less energy to compress so less energy returned.
But wait! Petrol has its throttle valve closed so there isn't as much air there to compress and infact the petrol engine will be operating in a partial vacuum cycle while the throttle valve is closed. Thats what it gives its engine braking, working against vacuum.

to be more simple if you have two different fly wheels, one is heavy and the other is lighter, will they need same amount of energy to turn both at same rpm? the heavier one needs more right?

>> Heavy needs more energy and heavy will store more energy and heavier the flywheel lesser the engine braking. Be it petrol or Diesel.

the energy you utilized for compression stroke will be dissipated in the next stroke and eventually null in the exhaust stroke. one more thing that many people believe that diesels doesn't have throttle valve and its not true here. an old matador engine features throttle valve. yes please check it out. modern diesels do have throttle flap and it all depends on OEMs to have or not.

>> Cool info there. So Matador engine with throttle valve will engine brake, because it has a throttle valve. Any engine with throttle valve will engine brake. But most Diesel engines do not use a throttle valve.

>> A butterfly valve in the throttle body is supposedly bad for the efficiency of the engine, be it petrol or Diesel (but Petrol can't do without it so its there). Throttle valve sits there always hurting the flow. This is why Diesels have jake and exhaust brakes, during normal operation it does not affect efficiency of the engine.

>> Diesel engines are more efficient because it uses high compression & compression ignition, diesel fuel have more energy stored in it, diesel engines have better flow & diesel engine have minimal pumping losses. Throttle valve in this hurts flow and pumping loss.

the usage of throttle flap is not only for engine braking but for controlling emissions too. certain OEMs prefer to use throttle flap for engine braking instead of a exhaust flap. and both the flaps are not always used for engine braking.they are more useful in EGR mixing.

>> Nice info there. Any such engines that you know of using throttle valve?

exhaust flap offer a mild resistance in the exhaust stroke. only suitable for light duty trucks.

>> No. Its also being used in tractor trailers, many places do not allow the use of jake brakes due to the noise it makes.

jake brakes or technically 'decompression valves' will offer one more stroke and that's the power stroke. jake brakes are more effective and only used on heavy duty trucks.

>> Jake brake do not offer one more power stroke. Jake brake bleeds the power stroke.

>> Jake brakes are used in lighter trucks too, not just heavy vehicles.

a small table explaining the techniques and its usage.

Attachment 281448

usually the heavy duty trucks will have combination of the above techniques for effective control.

>> Yep.

and one more techniq is the VGT. using VGT for engine braking. how? just simple, VGT fins are fully open and this offers back force in the exhaust stroke and as well as pumps more air into the cylinder which requires more force to compression the same.

>> Yes, its simple. Its just an exhaust brake but done in trucks with VGT. Resistance provided by a flap in the exhaust pathway is being provided by turbo vanes.

one more factor you guys need to think of, 'engine displacement is to weight of the vehicle' factor apart from gearing ratios.

the problem with the safari is governor calibration.

Originally Posted by DirtyDan (Post 1716985)

Diesels have less engine braking exactly because of their higher compression. The higher compression of a diesel provides far more force to the DOWNSTROKE of the piston than a petrol piston of the same displacement and therefore much more force turns the crankshaft and then the drive shaft etc. than is the case with a petrol engine. What a jake break does is let the air out of the balloon, let the air out of the DOWNSTROKE by venting it out the exhaust valves. This takes force away from the crankshaft and hence the rest of the vehicle. It is the DOWNSTROKE that is the real difference maker in engine braking between diesel and petrol along with the fact that the intake is open on the diesel on the upstroke and closed on the petrol.

I think it is the gearing that makes off-roaders think their diesels brake better than petrols.

Jake brakes were invented to save brakes on heavy vehicles. They might be useful in slowing the free fall of rogue planetoids like the Tata Safari.

Originally Posted by tsk1979 (Post 1717376)

The reason for better engine braking in petrols is the "throttle valve". No such contraption in diesels.

Originally Posted by headers (Post 1717436)

Not really.. Modern CRDi engines have throttle valves and ensure fuel cut off with respect to accelarator position IMO. The CRDi pump takes care of it alongwith the signal from the ECU.

Originally Posted by SPIKE ARRESTOR (Post 1717484)

@Headers, Where do CRDi engines have throttle valves? The solenoid switch of injectors gets pulse signals depending on the feedback given by the sensors, this signal is processed according to an algorithm preset in the ECU the ouput is then passed on to the injectors which act as an actuator. The fuel pump (high pressure pump) maintains a continuous pressure head inside the common rail (close to 1600 bar) irrespective of the acc pedal position. Hope it is clear now,

Spike

Originally Posted by tsk1979 (Post 1717560)

@headers, I think SPIKE_ARRESTOR is a CRD engine guy, regularly experimenting and tweaking engines. Correct me if I am wrong SPIKE_ARRESTOR?

Originally Posted by headers (Post 1717547)

Spike, Thanks for the clarification.

What sensor gives feedback to the solenoid? The accelerator position sensor?

Maybe I'm wrong in using "throttle valves" in modern terminologies. Me bad..

You are correct that the pressure head inside the common rail is 1600 odd bar. Again how does the injector open and close? Due to the electrical switching solenoids.

The soleniods get their input from the ECU which looks upon the reference table to tell the injector to open or close!

sorry for OT - Have you opened a injector of a modern CRDi Engine?

Cheers

Originally Posted by SPIKE ARRESTOR (Post 1717714)

i have seen a opened injector of a CRDe engine though not opened it myself.....

Accelerator pedal position sensor

Spike :D

Originally Posted by siddartha (Post 1717833)

No clear answer still....as usual :)

anyway this is what i think. not gonna get into any diesel/petrol, DI/IDI/CRDI etc etc discussion.

i think engine braking is more a funtion of the gearing/ gear ratios and the engine rpm. (and maybe vehicle weight and load on the vehicle)

lets say i am doing 100 kmph in fifth gear in a vehicle, i let go of the accelarator , i will not be getting any engine braking. ONLY when i start to downshift will i get any engine braking.