Why don't diesel engines pop and bang like petrol engines?
Discussion
I was out in my obnoxious RS200 track toy last night for a little drive and I got thinking, why can't diesel engines pop and bang like a petrol car?
Is it something to do with how diesel fuel is burnt under compression, rather than ignition for petrol?
I know people can have a 'pop and bang' remap for a diesel, but that seems more of a hard cut limiter to me, but I could be totally wrong here.
I'd be interested to know your thoughts
NG
Is it something to do with how diesel fuel is burnt under compression, rather than ignition for petrol?
I know people can have a 'pop and bang' remap for a diesel, but that seems more of a hard cut limiter to me, but I could be totally wrong here.
I'd be interested to know your thoughts
NG
Sounds like you have the right answer to me. Petrol pops and bangs are unburnt fuel igniting in the exhaust aren't they? Be that from the heat of the exhaust or retarded ignition timing or whatever.
Diesel won't auto ignite in a hot exhaust anyway, and diesel EGTs tend to be lower than petrol too.
Like you said I've only heard pops and bangs on a 'popcorn' limiter on a diesel.
Diesel won't auto ignite in a hot exhaust anyway, and diesel EGTs tend to be lower than petrol too.
Like you said I've only heard pops and bangs on a 'popcorn' limiter on a diesel.
Pops and bangs are produced by unburnt petrol hitting the hot exhaust. Unburnt diesel in the exhaust just leads to an oily exhaust and black smoke.
Edited to add, proper pops and bangs are made by older cars with carbs or relatively primative fuel injection systems. The modern ECU driven pops and bangs are a poor substitute.
Edited to add, proper pops and bangs are made by older cars with carbs or relatively primative fuel injection systems. The modern ECU driven pops and bangs are a poor substitute.
Edited by ecs0set on Thursday 23 September 08:52
A petrol engine has too low a compression temperature to auto-ignite it's fuel (petrol) so (normally) it requires a spark to do that, provided by the ignition system. When that spark is not their, or very late in the cycle (retarded) the fuel therefore goes into the chamber, gets vapourised but not ignited, and comes out the exhaust port with the fresh air charge it went into the cylinder with. That unburnt vapour then travels down the exhaust to hot components their, like the turbo or catalyst, where it has long enough (and is rich enough a mixture due to un-even mixing) to heat up to an auti-ignition temperature or is lit-off by hots spots in those exhaust line components. for examply the red hot leading edge of the catalyst substrate. It therefore ignited and burns, and because the burn is at a low pressure and highly caotic, you get the characteristic pops, bangs and whummps.
A diesel engine is running always with excess air and has a fuel and compression ratio that lights and burns the fuel as it is injected, so the fuel will always burn in the chamber. To get a modern derv to pop and bang is possible by retarding the injection timing to inject fuel at too low a chamber pressure, but because the fuel has a slow flame propagation speed (slow burn rate which is also what limits the maximum speed of a diesel engine) it tends not to go "bang" as such, more burn in a smokey, soft sort of way.
A diesel engine is running always with excess air and has a fuel and compression ratio that lights and burns the fuel as it is injected, so the fuel will always burn in the chamber. To get a modern derv to pop and bang is possible by retarding the injection timing to inject fuel at too low a chamber pressure, but because the fuel has a slow flame propagation speed (slow burn rate which is also what limits the maximum speed of a diesel engine) it tends not to go "bang" as such, more burn in a smokey, soft sort of way.
ecs0set said:
Edited to add, proper pops and bangs are made by older cars with carbs or relatively primative fuel injection systems. The modern ECU driven pops and bangs are a poor substitute.
Yes and no. Yes, almost all the "crackle" maps are a bunch of idiotic showing off *but* while EFI can cut fuel entirely on a closed throttle (and that is obviously better for emissions and economy) that does mean there is a momentary lag when you go from a closed to open throttle. So keeping injecting a bit and you improve that immediate throttle response.
It is admittedly more noticeable on bikes than cars but if you're after something with razer sharp throttle response then it could make a difference.
Fastdruid said:
It is admittedly more noticeable on bikes than cars but if you're after something with razer sharp throttle response then it could make a difference.
Given that almost everything with the stupid crackle maps is turbocharged, I think a few tens of ms while the injectors sort themselves out is the least of the problems with it comes to throttle response. 
kambites said:
Fastdruid said:
It is admittedly more noticeable on bikes than cars but if you're after something with razer sharp throttle response then it could make a difference.
Given that almost everything with the stupid crackle maps is turbocharged, I think a few tens of ms while the injectors sort themselves out is the least of the problems with it comes to throttle response. Also while being turbocharged means that immediate full power isn't on the table it does still make a difference. With turbocharging it would also mean that the turbo starts to spin up (slightly) earlier and hence give more power earlier.
Sure, we're talking almost miniscule differences, I think you'd be very hard pressed to notice the difference in a car (far more obvious on a bike) and even more so when turbocharged but that doesn't mean there isn't a difference.
With a carb, you're pulling air and fuel mix before you get to the cylinder. If you shut the throttle quickly, you mess up the air/fuel mixture briefly as the fuelling "catches up" with the engine demand (I'm not explaining that well) resulting in temporary overfuelling on the overrun giving you excess unburnt fuel in the exhaust. The point is, you have air AND fuel mixture early on in the process.
With a diesel, you suck in an amount of air only THEN add fuel. So the difference between what the engine demand and what the fuelling system is trying to supply is regulated in a different way so doesn't have the issues you have with a carburettor.
The difference is in the shortcomings of the fuelling mechanism.
With a diesel, you suck in an amount of air only THEN add fuel. So the difference between what the engine demand and what the fuelling system is trying to supply is regulated in a different way so doesn't have the issues you have with a carburettor.
The difference is in the shortcomings of the fuelling mechanism.
Networkgeek said:
I was out in my obnoxious RS200 track toy last night
Wait... You were driving a legendary Group B homologation special and wondering about diesels? 
That's like being up to your nuts in the Hollywood A-lister of your choice then wondering if the big lady on checkouts in your local supermarket does this too.Sod the rest of the post - what about the RS200? Replica or factory car? Original running gear? How what when why?
donkmeister said:
Networkgeek said:
I was out in my obnoxious RS200 track toy last night
Wait... You were driving a legendary Group B homologation special and wondering about diesels? That's like being up to your nuts in the Hollywood A-lister of your choice then wondering if the big lady on checkouts in your local supermarket does this too.Sod the rest of the post - what about the RS200? Replica or factory car? Original running gear? How what when why?
One of these.
Not one of these.
Fastdruid said:
donkmeister said:
Networkgeek said:
I was out in my obnoxious RS200 track toy last night
Wait... You were driving a legendary Group B homologation special and wondering about diesels? That's like being up to your nuts in the Hollywood A-lister of your choice then wondering if the big lady on checkouts in your local supermarket does this too.Sod the rest of the post - what about the RS200? Replica or factory car? Original running gear? How what when why?
Saying you drive an RS200 on a motoring site without mentioning the Clio is like telling your kids you're going on holiday to California, and then rocking up to a caravan park just up the coast from Great Yarmouth.
donteatpeople said:
Fastdruid said:
donkmeister said:
Networkgeek said:
I was out in my obnoxious RS200 track toy last night
Wait... You were driving a legendary Group B homologation special and wondering about diesels? That's like being up to your nuts in the Hollywood A-lister of your choice then wondering if the big lady on checkouts in your local supermarket does this too.Sod the rest of the post - what about the RS200? Replica or factory car? Original running gear? How what when why?
Saying you drive an RS200 on a motoring site without mentioning the Clio is like telling your kids you're going on holiday to California, and then rocking up to a caravan park just up the coast from Great Yarmouth.
ecs0set said:
Pops and bangs are produced by unburnt petrol hitting the hot exhaust. Unburnt diesel in the exhaust just leads to an oily exhaust and black smoke.
Edited to add, proper pops and bangs are made by older cars with carbs or relatively primative fuel injection systems. The modern ECU driven pops and bangs are a poor substitute.
Ahhh that makes so much sense. I did wonder why an older diesel would emit black smoke and it makes sense it's unburnt fuel. Edited to add, proper pops and bangs are made by older cars with carbs or relatively primative fuel injection systems. The modern ECU driven pops and bangs are a poor substitute.
Edited by ecs0set on Thursday 23 September 08:52
Fastdruid said:
donteatpeople said:
Fastdruid said:
donkmeister said:
Networkgeek said:
I was out in my obnoxious RS200 track toy last night
Wait... You were driving a legendary Group B homologation special and wondering about diesels? That's like being up to your nuts in the Hollywood A-lister of your choice then wondering if the big lady on checkouts in your local supermarket does this too.Sod the rest of the post - what about the RS200? Replica or factory car? Original running gear? How what when why?
Saying you drive an RS200 on a motoring site without mentioning the Clio is like telling your kids you're going on holiday to California, and then rocking up to a caravan park just up the coast from Great Yarmouth.
donteatpeople said:
Fastdruid said:
donkmeister said:
Networkgeek said:
I was out in my obnoxious RS200 track toy last night
Wait... You were driving a legendary Group B homologation special and wondering about diesels? That's like being up to your nuts in the Hollywood A-lister of your choice then wondering if the big lady on checkouts in your local supermarket does this too.Sod the rest of the post - what about the RS200? Replica or factory car? Original running gear? How what when why?
Saying you drive an RS200 on a motoring site without mentioning the Clio is like telling your kids you're going on holiday to California, and then rocking up to a caravan park just up the coast from Great Yarmouth.
I can safely and sadly say, I was talking about a Renault Clio RS200 cup, not the awesome Ford RS200.
ecs0set said:
Pops and bangs are produced by unburnt petrol hitting the hot exhaust. Unburnt diesel in the exhaust just leads to an oily exhaust and black smoke.
Edited to add, proper pops and bangs are made by older cars with carbs or relatively primative fuel injection systems. The modern ECU driven pops and bangs are a poor substitute.
Eh? RS3 and old pre OPF M5 sound great.Edited to add, proper pops and bangs are made by older cars with carbs or relatively primative fuel injection systems. The modern ECU driven pops and bangs are a poor substitute.
Edited by ecs0set on Thursday 23 September 08:52
TX.
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