wizzer
Road Burner
Why do diesel engines have so much Torque?
Could petrol engines be tuned the same way to give loads of low end torque and power?
Could petrol engines be tuned the same way to give loads of low end torque and power?
Why do diesel engines have so much Torque
- Thread starter wizzer
- Start date
Aerial Andy944
The Torque Meister
Good question. It's something to do with the burn rate which is why they don't rev very high. I'm sure our diesel guru Hdi will be along soon to give a more detailed answer
Vw diesels - very bad 
HDi fun
TC ModFather
Waynne, it's not the VW engines I dislike, it's the thing they're bolted to, namely the rest of the car !!!
The primary reason for diesel engines' torque characteristics are due to high BMEP (Brake Mean Effective [cylinder] Pressure). Compression ratios are typically 18:1 or higher. Naturally aspirated diesels can run as high as 24:1 !! Turbo-charging does contribute enormously to the torque delivery, as it does with a petrol engine.
Burn rate forms part of it, too, but this is more to do with lower rev limits than low end shove. Put simply diesel does not burn quickly enough for the framefront to exert downward force on the piston crown as revs rise. The piston is effectively running away from the flamefront so this is a kind of self imposed rev limiter. Of course bore/stroke and many other factors are at work here but diesel does burn differently from petrol.
@crowley - diesel bad? That's a bit of a sweeping statement. What do you mean?
The primary reason for diesel engines' torque characteristics are due to high BMEP (Brake Mean Effective [cylinder] Pressure). Compression ratios are typically 18:1 or higher. Naturally aspirated diesels can run as high as 24:1 !! Turbo-charging does contribute enormously to the torque delivery, as it does with a petrol engine.
Burn rate forms part of it, too, but this is more to do with lower rev limits than low end shove. Put simply diesel does not burn quickly enough for the framefront to exert downward force on the piston crown as revs rise. The piston is effectively running away from the flamefront so this is a kind of self imposed rev limiter. Of course bore/stroke and many other factors are at work here but diesel does burn differently from petrol.
@crowley - diesel bad? That's a bit of a sweeping statement. What do you mean?
I was thinking more of the emissions scandal!
HDi fun
TC ModFather
It appears it only applies to post '09 models. I am still of the opinion that it's fuss over nothing. If the cars are driven to the test prescription then they will achieve the official figures. We all know that these things vary hugely according to usage and driving style.
I'm still struggling to understand the NOx issue anyway. What's the problem with NOx? It's just governmental nonsense. As yet we have no evidence that climate change exists anyway.
I'm still struggling to understand the NOx issue anyway. What's the problem with NOx? It's just governmental nonsense. As yet we have no evidence that climate change exists anyway.
The cars need to be driven with no steering inputs to achieve the economy and emissions tested for.
Yugguy
Torque King
Isnt nox poisonous tho? bad for the lungs?
HDi fun
TC ModFather
So is CO2 yet the planet depends upon the stuff. It's possible that we interfere too much. There's only so much fossil fuel left and once it's gone it's gone. Probably not enough to destroy the planet, so no problem there directly.
We should embrace nuclear in my opinion.
Nuclear cars, that would be awesome. Loads of power on tap!
HDi fun
TC ModFather
I hadn't quite got to the point of considering a reactor and steam driven generator under the bonnet but now you mention it .... 
Throw in a flux capacitor for good measure and you'll never be late again!
Aerial Andy944
The Torque Meister
Yeah you'd have loads of power.........once!! Use it wisely
Aerial Andy944
The Torque Meister
I'm quite liking my diesel (sorry Os) at the moment. Easy and refined just cruising aound and a big shove back into your seat out of the corners when you need to get a move on. Not as exciting as a screaming petrol engine but satisfying nonetheless. I must be getting old 
Ronbros
Torque Junkie
diesel engine torque is higher than petrol,it has more BTU energy, simply(like HDI said), because of its slower combustion burn rate, therefore expansion pressure pushing the piston, lasts longer on the power stroke!
(one of the reasons why hi-octane petrol burns slower,longer push time).
most petrol engines have spent/used effective pressure,by 90/100* crank angle after TDC.
where as diesel expansion pressure can be carried as far as 160/170* crank angle after TDC.
diesel has higher comp.ratio gives more expansion pressure, old design diesel engines had to be built heavy and strong, modern engineering(mainly EFI), has made them lighter and quieter!
(one of the reasons why hi-octane petrol burns slower,longer push time).
most petrol engines have spent/used effective pressure,by 90/100* crank angle after TDC.
where as diesel expansion pressure can be carried as far as 160/170* crank angle after TDC.
diesel has higher comp.ratio gives more expansion pressure, old design diesel engines had to be built heavy and strong, modern engineering(mainly EFI), has made them lighter and quieter!
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HDi fun
TC ModFather
I wouldn't say that they're any less strongly built, it's more of a case of petrol engines now being built to similar levels of ruggedness as a concurrent diesel engine. Historically diesel engines would outlive petrols, this is no longer the case.
Common rail EFI with multiple injection phases absolutely is the big contributor to refinement.
What Ronbros says about expansion pressure is bang on ( ). It's also the reason for diesel engines' much lower rev limit compared to petrol ones.
I like diesel's low revving shove, yet I think the flexibility of a petrol engine (especially with forced induction) probably is the better arrangement. Much easier to drive at low speed and free revving when required.
Common rail EFI with multiple injection phases absolutely is the big contributor to refinement.
What Ronbros says about expansion pressure is bang on (
). It's also the reason for diesel engines' much lower rev limit compared to petrol ones.I like diesel's low revving shove, yet I think the flexibility of a petrol engine (especially with forced induction) probably is the better arrangement. Much easier to drive at low speed and free revving when required.
Just noticed this.
Wrong, both burn. However, petrol will explode if the mixture or timing is wrong. You can hear it as a pinking noise.
HDi fun
TC ModFather
It's a horrible a noise. I owned a couple of Austin/Rover Montegos in late 1908s/early 1990s. They made that horrible rattly sound all the time. O series and S series engines weren't the best but even so it was not pleasant.
Just as long as Bethesda don't design them.
Cruddy things that explode as soon as you look at them funny...
Ronbros
Torque Junkie
what is Bethesda?
A video games company. They own the "Fallout" franchise, which is based on a 1950s view of the future, after a nuclear war between the US and China.
Link:
HDi fun
TC ModFather
It's also a town in North West Wales.
Ronbros
Torque Junkie
we have a Bethesda in USA ,its in state maryland.
sorry dont play any games, im lucky to just use a simple computer basics.
sorry dont play any games, im lucky to just use a simple computer basics.
Ronbros
Torque Junkie
a million years ago ,there were no humans, a million years from today ther wont be any either.
HDi fun
TC ModFather
We are all too war happy.
To be honest, unless society seriously changes, we'll be lucky if there's any humans a millenium from now.
Resulting from tribalism and or religion IMO
SLEEPER
Pro Tuner
- Points
- 592
So back to the topic ( it is a car forum)
It would be interesting to see how much low down torque a well tuned petrol engine could produce using one of the new generation turbos like a small borg warner efr or even baby twins
You would need to set the cam and head set up accordingly but it should be possible to get a real response from pretty low revs and get diesel response pretty easily.
Mind you just because you can make a chicken perform like a duck doesnt mean you should. Its probably easier to just go with the duck.
It would be interesting to see how much low down torque a well tuned petrol engine could produce using one of the new generation turbos like a small borg warner efr or even baby twins
You would need to set the cam and head set up accordingly but it should be possible to get a real response from pretty low revs and get diesel response pretty easily.
Mind you just because you can make a chicken perform like a duck doesnt mean you should. Its probably easier to just go with the duck.
Aerial Andy944
The Torque Meister
Now there's a quote I'm going to save on file, brilliant
HDi fun
TC ModFather
Diesels have the big low end torque due to high BMEP (Brake Mean Effective Pressure). Basically more force shoving pistons on the down stroke - power stroke - away from the combustion chamber.
Yugguy
Torque King
The c220 has a twin turbo setup, small high pressure for boost at lower rpm and a larger lower pressure one for boost further up the revs.
Ronbros
Torque Junkie
like i said earlier, Diesel fuel has more BTU energy in it ,like 25-30% more.
Ronbros
Torque Junkie
google BMW M3 petrol, VS Alpina diesel 3.
a good comparison of two almost identical BMWs
a good comparison of two almost identical BMWs
Yugguy
Torque King
I think there's a lot of snobbery about petrol vs diesel, like manual vs auto. Yes absolutely a petrol v8 or v12 sounds far better but isnt grunt just grunt?
I have to agree that I find many petrol snobs around will never go diesel even if on paper it is a better option for them.
Yugguy
Torque King
I deliberately went for diesel and will again purely because I like low down torque and anything above 3k seems too high to me.
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