Diesels, torque and power

[bella]

Originally Posted by tristancliffe

Why do some people think there is a special technique for driving diesels. There isn't. You just work with a lower rev range, and change gear when you have more tractive effort in the next gear than the current gear, just like a petrol. They are more tolerant of lower revs than petrols because of the low rpm torque, but that's about it, and doesn't require a change of driving technique.

already there you've gone far beyond the vast majority's understanding of how to drive a car. a lot of people who call themselves competant rag diesels senseless petrol stylee when that's simply not the best way to drive them. it takes a slight understanding of how the engine and turbo function to get the best acceleration - for example i find that unlike petrol, it's best NOT to floor the accelerator in a diesel with a turbo.

[tristancliffe]

Originally Posted by blackx

How so

Because the main advantage of a diesel is the cruising fuel economy, which is because there is no pressure drop across a throttle in a diesel (no throttle), which means less work on the piston, which means improved efficiency. Not explained well, but that's the gist of it.

already there you've gone far beyond the vast majority's understanding of how to drive a car. a lot of people who call themselves competant rag diesels senseless petrol stylee when that's simply not the best way to drive them. it takes a slight understanding of how the engine and turbo function to get the best acceleration - for example i find that unlike petrol, it's best NOT to floor the accelerator in a diesel with a turbo.

The thing is you can feel the change - you know instinctively when the car is running out of puff because the seat of your pants (and the rate of increase of the speedo/rev counter) tells you. Trying the next gear to see if it's any better is a good thing to do, and after, say, 3 minutes you'll have a pretty good understanding of when to change gear.

But maybe I feel that because I know to look for it??

As for not using full throttle in a turbodiesel, how would that help? Are we talking from an economy point of view or a 0-62 point of view? If you are going for speed or acceleration surely more fuel (and it'll be metered automatically so you can't just dump too much in) equals more power = more heat = more boost = more power etc? Lower throttle (which is a misnomer in a diesel, but the pedal is still called the same thing! ) would make you slower but perhaps save fuel - although it is generally better to accelerate as fast as possible to your cruising speed (like a boy racer) rather than take a year to reach it (like a granny) from an economy point of view, assuming you don't have to suddenly slow down again for a roundabout or traffic light...

So (and I don't have a VAST amount of diesel experience, even though I drive a couple fairly regularly) I would have said that more throttle = a good thing. And it masks the diesel clatter quite well too

[bella]

on the throttle thing, i'm not familiar with all that technical stuff (excuse the blondeness ), i just know how the two diesels i drive work, but full throttle when the revs aren't quite high enough for the turbo to kick in has the same effect as say, half throttle. the result tends to be profanity, a gear change and the cloud of smoke though

i balance economy and outright speed though rather than just go for speed, which i think is the mindset of a lot of diesel drivers and a difference from most petrol drivers - if you want speed, surely you buy something manly like a v6 petrol something or other with a big growl... or a 330d. and if you're not bothered, you go for an unleaded because it sounds better, and the fuel is cheaper...

i guess when the car gets older it's easier to tell when the turbo kicks in and when to work with it. and it's really difficult to remember what you do as an instinctive reaction when you're driving!

[blackx]

Originally Posted by tristancliffe

Because the main advantage of a diesel is the cruising fuel economy, which is because there is no pressure drop across a throttle in a diesel (no throttle), which means less work on the piston, which means improved efficiency. Not explained well, but that's the gist of it.

Speaking as a diesel mechanic of some 20+ years that still doesnt explain why the diesel loses its advantage as stated here

Originally Posted by tristancliffe

And once petrol engines are de-throttled diesel will lose most of it's advantages in the steady cruise.

[tristancliffe]

Yes it does. The advantage of diesel is the lack of throttle. When petrol becomes de-throttled (which is what a lot of research is going towards) then neither has an advantage in this regard. At which point the popularity of diesels in all but the commercial (requiring lots of torque) sector will drop a lot (and people will see how bad they are environmentally!)

[terence]

I wonder if the AUDI LM team are aware of this.

[tristancliffe]

Probably. That's why they only entered a diesel when the equivalency rules made it stronger. With equal regulations I'm afraid that petrol remains quicker.

But the Audi thing is more about marketing and image anyway.

[terence]

Well it did the trick see-ing as other regular Entrants are going the diesel route.

[tristancliffe]

But as the playing field isn't quite level with the different fuels, it's not fair to say that diesels are suddenly better. They aren't (though I don't doubt it didn't improve the technology a bit). A lot of it is political - some nonsense about diesels being better for that nonsense about global warming.

[terence]

I quite agree,the smut situation cannot be that good,thats why some parts of London are trying to limit Trucks.

[blackx]

Originally Posted by terence bower

I quite agree,the smut situation cannot be that good,thats why some parts of London are trying to limit Trucks.

Thats all to do with swelling Kens coffers

[terence]

I was trying to be diplomatic

[blackx]

Originally Posted by terence bower

I was trying to be diplomatic

I am being diplomatic

[johnh875]

I think what Tristan is trying to say is the throttle butterfly provides drag/pressure drop in a petrol engine (except BMW Valvetronic presumably) and because a diesel does not have one it has an advantage. I knew a guy who had experienced this effect when he put a supercharger on his car. It was a 390hp V8 and the supercharger added probably another 100hp, but the surprising thing was that cruising fuel economy improved, which he put down to the intake air being pumped past the throttle body (v. low pressure at cruising only) rather than having to be sucked.