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Horsepower or Torque?
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Turbodiesels are torquier than petrols, and they're more economical, but they're not more economical because they have more torque. Rather, they're more economical because a litre of diesel has a higher calorific value than a litre of petrol.But doesn't that go against what everyone's been saying?
I have to rev the sh*t out of my petrol car where it guzzles juice, whereas the diesel one with the same bhp never has to be put above 3,000 revs to accelerate at the same rate.
What gives Anan?
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Turbodiesels are torquier than petrols, and they're more economical, but they're not more economical because they have more torque. Rather, they're more economical because a litre of diesel has a higher calorific value than a litre of petrol.
True but they are also more efficient than petrols to I begin with.0 -
Turbodiesels are torquier than petrols, and they're more economical, but they're not more economical because they have more torque. Rather, they're more economical because a litre of diesel has a higher calorific value than a litre of petrol.
But if you take the diesel/petrol calorific value out of it for a second, wouldn't the former still be more economical than the latter because you're revving her less?
(Unless calorific value actually means you don't have to rev diesels as much and in that case disregard what I just said
) 0 -
AFAIK no. Let's say an engine puts out peak torque at 2k rpm, and you're going up a hill in 3rd at 60km/h at 2k rpm, If you then changed up to 4th the revs would fall but your fuel consumption would actually rise because the engine was no longer at its most efficient. I'm sure it's more complex than that, and I'm not an engineer, but fuel consumption isn't directly tied to rpm.But if you take the diesel/petrol calorific value out of it for a second, wouldn't the former still be more economical than the latter because you're revving her less?
(Unless calorific value actually means you don't have to rev diesels as much and in that case disregard what I just said)0 -
But if you take the diesel/petrol calorific value out of it for a second, wouldn't the former still be more economical than the latter because you're revving her less?
(Unless calorific value actually means you don't have to rev diesels as much and in that case disregard what I just said)
A Diesel will be more economical than a petrol. That is the only reason that it exists. Note the capital D for Rudolf Diesel. He set out to build a more economical engine.
To do this he followed basic physics. He needed to get as much of the heat generated by burning the fuel out of the burned mixture before throwing most of it away via the exhaust.
In order to extract as much mechanical energy as possible he needed the greatest possible expansion ratio. Given the he needed the greatest possible expansion ratio he needed the highest possible compression ratio.
The high compression meant the the fuel could not be mixed with the air before compression or pre ignition would occur, hence the fuel injection.
It is inherently more efficient than the petrol engine, also inherently more expensive to build when you compare the basic form of each.0 -
But doesn't that go against what everyone's been saying?
I have to rev the sh*t out of my petrol car where it guzzles juice, whereas the diesel one with the same bhp never has to be put above 3,000 revs to accelerate at the same rate.
What gives Anan?
I did say, "assuming equal-ish crank-throws and whatnot"! No, what I'm trying to come at really is that the genesis of torque is setting fire to a load of explosive gas and letting it flake a piston down against a load. The more of that gas you set fire to (and the stoichoimetrically better the mix, but that's a whole 'nother tin a' beans) the more torque. In short, our cousins actually have a point when they say, there ain't no substitute for inches, and two litres is a goddam soda bottle! 0 -
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Torque is a measure of twisting force. A torque of 1 Nm applied to a lever 1 m long will exert a force of 1 N at the other end of the lever as it rotates (or attempts to rotate). If you make the lever shorter it will exert more force for a given amount of torque (this is how gears work).
In a car, torque gets turned into tractive effort at the wheels i.e. a force exerted by the wheels on the road which tries to push the car forward. If the force applied by the wheels is different to the forces acting against the car, it accelerates due to Newton's Second Law (F=ma) (this can include deceleration).
But this is where it gets a bit complicated. Because acceleration changes the speed of the engine too, the torque available may change. This is where power comes in: it tells us how the torque changes with time (which is directly linked to engine speed). This will tell us how the rate of acceleration changes.
An engine with a flat torque curve will have a power curve that rises linearly at 45 degrees. This engine will accelerate a load at a constant rate (provided things like aerodynamic drag are ignored
).
Maximum power will always occur at higher revs than maximum torque since it's a function of engine speed and torque.
On the fuel efficiency thing:
More torque always equals more fuel, all other things being equal. It's demanding more energy be used so that energy must come from somewhere. Likewise, more power means more fuel.
It's important to distinguish between engine speed and engine load. Revving the car to the redline in neutral will send the engine speed way up, but at very low load since there's nothing for it to push against. 1500 rpm in 1st at full acceleration is going to use up much more fuel since the engine has a much higher load to work against (shoving 1.5 tonnes of car along takes a bit of effort).
Load is the main driver of fuel consumption. The engine controller meters out fuel based on demand for power, not engine speed.
Why do some engines deliver more power/torque for a given level of fuel consumption? Efficiency.
The compression ignition engine is inherently better at turning fuel into power than the spark ignition one. It's a thermodynamic certainty.
On top of that you add volumetric efficiency which depends on the internal design of the engine, particularly intake, head and exhaust. You've also got friction in the moving parts working against you.
Then there's injector design and combustion efficiency. Finer sprays and better control means fuel burns faster and more completely. This is especially why diesel engines have improved so much in the last 20 years. Incidentally, this is where the rev limit is imposed on diesel engines too: diesel can only burn so fast and raising the rev limit just produces smoke instead of power. The reason LeMans diesels have been able to rev harder is down to high tech synthetic fuels with very high cetane numbers (72 vs ~48-50 for road fuels).
The last thing is ancillaries: alternator, fuel pump, water pump, power steering pump, aircon compressor, etc. They all take their cut.0 -
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