hairychris21 Posted November 9, 2011 Share Posted November 9, 2011 Right. This is odd... In my experience, and probably yours too, diesels provide significantly more engine braking than the equivalent petrol engine. This has always been a fact so obvious as to not even need questioning. However, having daftly got into a bit of a discussion about this on a non LR forum, (STW) it appears there are lots of very plausible theories that state that diesel do not have any real engine braking, and certainly less than a petrol engine. Even Wikipedia states the same, allegedly written by a 'scientist'. Here's the thing, I know I'm right, but why? Compression ratios? I used that argument, but they came back with the fact that the air compressed also decompresses on the down stroke, negating the loss. Wrong, but plausible. Help! Either feel free to pop over to STW and join in, or give me something I can take back with me.... Cheers guys! Quote Link to comment Share on other sites More sharing options...
Rich_P Posted November 9, 2011 Share Posted November 9, 2011 In my limited experience, the only engine that has appeared to have similar engine braking to a typical diesel is the Rover V8 I think, but that's probably because it's got something to do with twice as many cylinders compressing air? Quote Link to comment Share on other sites More sharing options...
C18RCH Posted November 9, 2011 Share Posted November 9, 2011 STW? Got a link? Quote Link to comment Share on other sites More sharing options...
hairychris21 Posted November 9, 2011 Author Share Posted November 9, 2011 http://singletrackworld.com/forum/topic/driving-and-the-use-of-brakes Posting on my phone so don't know if it'll show up as a link, if not just cut n paste into address bar... V8; absolutely, unless on an auto... ;-) I'm V8ninety on there, unless you were wondering! Quote Link to comment Share on other sites More sharing options...
discomikey Posted November 9, 2011 Share Posted November 9, 2011 if youre on over run, an engine is still trying to compress the air in the cylinders, but not really powering the crank to overcome the compression. on a petrol there is less of a compression ratio which means the engine can spin more freely than the equivalent diesel. how about you invite them for a ride in your landy with 3.5 tons on the back and then do the same in a petrol? im sure theyll feel safer going downhill in the diesel. diesel engine braking was the cause of Brians write off so i know it works... Quote Link to comment Share on other sites More sharing options...
zardos Posted November 9, 2011 Share Posted November 9, 2011 There are probably so many factors on how the engine is fuelled and controlled on different engines petrol or diesel that a certain petrol could have less engine braking that another diesel or the other way around. But I think that the petrol has more control over when combustion happens (with the spark) than a diesel so it would be possible to advance the combustion to such a point where it is before TDC and producing a gas expansion force against the normal direction of movement of the piston, thus slowing the engine more (ps don't know if any petrol engine does this, so might be talking rubbish ) Quote Link to comment Share on other sites More sharing options...
hairychris21 Posted November 9, 2011 Author Share Posted November 9, 2011 it would be possible to advance the combustion to such a point where it is before TDC and producing a gas expansion force against the normal direction of movement of the piston, thus slowing the engine more (ps don't know if any petrol engine does this, I think this is pre ignition or pinking, and generally considered to be a 'bad thing'! Lots of nasty shock loads through your bearings... I think your point about different engines is spot on though, most of these cycle riding muppets (thats all of them with a different view to THIS cycle riding muppet) on the other forum probably drive ultra modern gutless euroboxes, all lectronic and stuff and probably rely on a sticker to even tell them the difference between petrol and diesel! Quote Link to comment Share on other sites More sharing options...
mmgemini Posted November 9, 2011 Share Posted November 9, 2011 If you read Mike Hawthorns book about his racing, you''ll see a comment that the 3liter 4 cylinder Ferrari had good engine braking. The 3litre V12 on the other hand had no engine braking. So will piston size also come into the equation. Quote Link to comment Share on other sites More sharing options...
disco_al Posted November 9, 2011 Share Posted November 9, 2011 If you read Mike Hawthorns book about his racing, you''ll see a comment that the 3liter 4 cylinder Ferrari had good engine braking. The 3litre V12 on the other hand had no engine braking. So will piston size also come into the equation. according to that, then yes, piston size will make a difference, as the v12 had a cyclinder volume of 250cc compared to 750cc for the 4 pot Quote Link to comment Share on other sites More sharing options...
discomikey Posted November 9, 2011 Share Posted November 9, 2011 it would be possible to advance the combustion to such a point where it is before TDC and producing a gas expansion force against the normal direction of movement of the piston, thus slowing the engine more (ps don't know if any petrol engine does this, ignition is usually around 8degrees BTDC anyway, any further forward= pinking as said above and possible damage to engine. Quote Link to comment Share on other sites More sharing options...
TSD Posted November 9, 2011 Share Posted November 9, 2011 I think it's due to the change in pumping losses, due to throttle plate and compression ratio. When you're on the over-run in a diesel, your still compressing a full cylinder charge of air on each compression, since there's no throttle plate. So the cylinder pressure will be up around 20 bar at the top of the stroke. On a petrol engine, you'll have the throttle plate shut on the over-run, so you don't allow the cylinder to fill properly in the first place, then only compress it by around 10:1. So cylinder pressure is probably below 4 bar just before ignition. (Ricardo refers to petrol engines as variable compression ratio engines for this reason) In the very earliest incarnations of MS with EDIS, I tried playing with overrun timing to improve engine braking on my TBI Ford V6. I had the idea it might be better to fire slightly ATDC to waste as much of the power stroke as possible, but I never really had chance fully investigate. (I took the truck apart, and it's still in bits ) Quote Link to comment Share on other sites More sharing options...
Range Rover Blues Posted November 9, 2011 Share Posted November 9, 2011 Engine overun on a petrol causes some nasty emissions, so often it's controlled. Ealry on with dampers on the throttle and now by drive-by-wire systems. How do you decide what the equivelent petrol and deisel engine is? by capacity, by power? On the LR the TDi has a lower 1st gear, this will make a difference. Several manufacturers of trucks use engine braking devices, they have been around since the 30's and restrict the flow of exhaust gas, turning the engine into a big pump that absorbes lots of energy. On a car, I'd hate to say. I think the gearing on the LR is what makes the difference. My micra DCi has little engine braking compared to it but about the same as a 1300 Fiesta. Unlike a petrol though the fuel pump cuts the fuel on over-run, not all petrol engines do that. To answer it definitively would need a dynomometer. Quote Link to comment Share on other sites More sharing options...
zardos Posted November 9, 2011 Share Posted November 9, 2011 And the difference between the 1300 Fiesta and a TDi is the weight of the vehicle that the engine is trying to slow. Quote Link to comment Share on other sites More sharing options...
Range Rover Blues Posted November 10, 2011 Share Posted November 10, 2011 No, I mean the microbe is about the same as the Festa, but I don't off-road either and don't use the gears to slow down much either. Quote Link to comment Share on other sites More sharing options...
Off Road Toad Posted November 10, 2011 Share Posted November 10, 2011 I've had various engines in my 90" - It started life as a 300 tdi, engine braking was excellent. I put a 3.9 RV8 high compression engine in it and noticed engine braking was not as good. I blew this engine up and replaced it with a low comp 3.9, This was a bad move, amonst other things engine braking was non exsistant. This engine was replaced with a 4.6 and engine braking was as good as the 300 tdi although it changed about with the different maps i used in megasquirt. I now have a tuned td5 in the same car with the same gearing and engine braking is much improved over any of the previous engines. Just what i've experienced from the different options all in the same car. Steve Quote Link to comment Share on other sites More sharing options...
Soren Frimodt Posted November 10, 2011 Share Posted November 10, 2011 I think it's due to the change in pumping losses, due to throttle plate and compression ratio. When you're on the over-run in a diesel, your still compressing a full cylinder charge of air on each compression, since there's no throttle plate. So the cylinder pressure will be up around 20 bar at the top of the stroke. On a petrol engine, you'll have the throttle plate shut on the over-run, so you don't allow the cylinder to fill properly in the first place, then only compress it by around 10:1. So cylinder pressure is probably below 4 bar just before ignition. (Ricardo refers to petrol engines as variable compression ratio engines for this reason) In the very earliest incarnations of MS with EDIS, I tried playing with overrun timing to improve engine braking on my TBI Ford V6. I had the idea it might be better to fire slightly ATDC to waste as much of the power stroke as possible, but I never really had chance fully investigate. (I took the truck apart, and it's still in bits ) I believe you are spot on! If you ever tried testing compression on a petrol engine with the throttle open and closed, you will know the difference. Another thing is the fact a petrol engine will combust with even the slightest amount of petrol in the mix, meaning that if you compare a carburetted petrol engine with a diesel, the doesn't completly shut off the mixture as the diesel does when on overrun. I think these are the reasons Quote Link to comment Share on other sites More sharing options...
Team Idris Posted November 10, 2011 Share Posted November 10, 2011 My first thought; Compression won't have much effect becuse the pressure is there to push the piston back down on what could have been the power stroke if diesel fuel was present. Its just an air spring. But; compressing air creats heat which is passed into the cylinder walls and oil. The crank turning force is turned into heat energy, the same as the brakes turn the rotating energy into heat via friction. Proof that above must be true is that energy cannot be destroyed or created, only moved around. So 'engine braking' must turn watts mechanical into watts "somthing" and heat sound favorite. Diesel does more air compressing so it'll be the better brake As a footnote, a flap/valve in the inlet manifold in a compressor is a 'suction offloader' which creates a vacuum. The compressor runs 'free' as compressing nothing takes no energy! For extra pub-quiz knowledge; a 3kW electric compressor produces 3kW of heat. It's as though you compress the air by pumping the heat out of it ! Quote Link to comment Share on other sites More sharing options...
zardos Posted November 10, 2011 Share Posted November 10, 2011 Compression is only 1/4 of the cycle, so may be a diesel might provide more braking on the compression cycle, but the petrol probably provides more braking on the suck part of the cycle. Too many factors to make a generic statement Quote Link to comment Share on other sites More sharing options...
TheRecklessEngineer Posted November 10, 2011 Share Posted November 10, 2011 I think Team Idris is on to the right track... A diesel engine compresses a full charge of air - this creates a lot of heat. If none of this heat escaped, then the piston would receive the same force going down as it did going up. However, there is quite a lot of cylinder cooling going on - you've got jacket cooling, and piston cooling via the oil. You've also got the massive swirl of the compressed air that is normally used to ensure complete combustion further encouraging heat transfer. Therefore much of this compression generated heat will be removed (this is the reason why some diesel require glow plugs to start - so much heat is removed on the compression stroke by a cold liner/head that it is below the auto-ignition temperature of the fuel). On the downward stroke then the piston will not receive nearly as much energy as it put in. Therefore: Engine braking. I forget the thermodynamic names for these processes - I've got a vague recollection of something like adiabatic compression and expansion, but I might be thinking of thunderstorms. Brownie points for anyone who knows the right names. There are also friction losses to consider. The largest frictional loss in an engine is the piston vs liner. So an 8 cylinder engine will have twice the losses as a 4 cylinder engine (assuming comparable piston sizes). However, tighter (and mostly more numerous) rings in a diesel vs a petrol will increase friction losses in favour of the diesel. If you also consider that everything in a diesel is heavier - it's likely that the losses from the bearings in a diesel will outweigh those from a petrol (although I'm not certain of this). And there's also the fuelling issue - a petrol engine on carbs will still be fuelling on overrun, so won't brake at all, whereas the governor on a diesel will stop fuelling on overrun (yes, even the fancy non-electronic ones). An EFI engine will cut fuelling on overrun though, so you need to be careful re like for like. So in short: Diesels will have better engine braking over petrols because of losses in compression and greater friction. Quote Link to comment Share on other sites More sharing options...
zardos Posted November 10, 2011 Share Posted November 10, 2011 I think the answer is send to Mythbusters Quote Link to comment Share on other sites More sharing options...
CwazyWabbit Posted November 10, 2011 Share Posted November 10, 2011 I think the answer is send to Mythbusters They'll just blow both engines up..... or shoot them! Quote Link to comment Share on other sites More sharing options...
cackshifter Posted November 11, 2011 Share Posted November 11, 2011 I have to say I'm with the Reckless Engineer on this. Ignoring size, volumetric and frictional differences between the various engine types ,we all know there's more compression in a diesel so the air gets hotter, hot enough to ignite diesel. So as it's hotter it transfers more heat to the coolant and there is warmer air leaving the cylinders. That rejected energy has to come from somewhere and its the crankshaft it comes from. Moreover, when a petrol engine is braking the throttle is closed, so it is not pumping much air, so this effect is even less whereas a diesel pumps at full throttle all the time, (that's why a hoover speeds up with your hand over the end, so no air can flow) Quote Link to comment Share on other sites More sharing options...
Range Rover Blues Posted November 11, 2011 Share Posted November 11, 2011 Not exaclty, a petrol engine pumps air against the restriction of the throttle plate, so it absorbs more energy, this is referred to a "pumping loss". Quote Link to comment Share on other sites More sharing options...
Team Idris Posted November 11, 2011 Share Posted November 11, 2011 Maybe a tiny bit of friction. But it sucks air through the throttle plate. If the volume is reduced it will take on heat energy from its surroundings. Thats why carbs used to freeze up. That's putting energy into the system, not expelling it. Quote Link to comment Share on other sites More sharing options...
TheRecklessEngineer Posted November 11, 2011 Share Posted November 11, 2011 Not exaclty, a petrol engine pumps air against the restriction of the throttle plate, so it absorbs more energy, this is referred to a "pumping loss". Not quite true. You'd only get a significant pumping loss if the working fluid was incompressible. Quote Link to comment Share on other sites More sharing options...
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