Why do diesel engines have so much Torque

HDi fun

TC ModFather
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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?
 

HDi fun

TC ModFather
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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.
 

Ronbros

Torque Junkie
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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!
 
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HDi fun

TC ModFather
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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.
 

HDi fun

TC ModFather
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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.
 

WLBjork

Road Burner
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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:
- about 18 mins in to see what I mean. Yes, those cars are nuclear powered...
 

SLEEPER

Pro Tuner
Points
562
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.
 

WLBjork

Road Burner
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I know the duck (DUKW).

Had to look up "Chicken Automobile".

Yep, the DUKW is much better as an amphib!
 

HDi fun

TC ModFather
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Location
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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.
 

obi_waynne

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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.
 

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