On the Diesel V Petrol debate...

But not *all* the dissipated heat is used to propel the car, in fact, depending on your definition of "dissipated", it might be the case that

Aside the noise of course.

Fairly arbitrary selection from:

1. The efficiency with which a heat engine transforms the potential heat of its fuel into work or output, expressed as the ratio of the useful work done by the engine in a given time interval to the total heat energy contained in the fuel burned during the same time interval, both work and heat being expressed in the same units.
2. = thermodynamic efficiency.

Thermodynamic Efficiency

In thermodynamics, the ratio of the work done by a heat engine to the total heat supplied by the heat source. Also called thermal efficiency, Carnot efficiency.

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Which you will notivce is rather different from your attempt to define thermal efficiency in: Message-ID:

but entirely in agreement with my own observations on thermal efficiency.

You must have to work hard to be so stupid that you post as proof a contradiction of your own statements.

Ah but then each cylinder is tiny, meaning you only get a teeny little explosion each time. In practice, the select few 2.0 V6s around (Rover's and Mazda's are all I can think of) make no more power than their 4-cyl counterparts.

Yep.

Nope. Do you ?

I don't care :)

Er, it's not :)

But a huge chunk of that heat goes to heating up the cylinder, and engine block itself. If ALL the heat from the fuel, heated the air in the piston, then it would get even hotter, so it would expand even more, so the expansion would force the piston down even harder, so you'd make more power, and it'd be more efficient :)

But it never is :)

But it usually doesn't. Any engine that did so, would be s**te.

Agreed. However, in practice, there isn't a vast amount of variance between the different lumps - certainly not enough to make up a large power defecit :)

The temperature of the gas is reduced as a result of doing work in moving the piston - some of the heat is converted to mechanical energy. The fraction of the heat energy which is converted to mechanical energy is the thermodynamic efficiency of the engine.

In the final maximum-entropy heat-death of the universe, all the energy released will have been converted to heat, and the net TE of any engine will be 0, however for the system comprising just the engine, as per the original discussion, the TE is given by (work out)/(heat in). The theoretical TE of a constant specific heat Diesel cycle with a compression ratio of 18 is approx. 70%. A practical diesel engine manages about 40%. The theoretical TE of an Otto cycle engine with a CR of 10 is about 59%. A practical petrol engine manages about 30%.

The calorific content of UL pterol is 35.34MJ/l, and of Diesel is 37.8MJ/l.

The net effect is that a petrol engine will use approx 42% (volume) more fuel per aribtrary power profile.

there is an optimum cylinder size afaik, f1 cars use it, its not very big i dont think.

The way this works, is basic thermodynamics. Energy can be neither created nor destroyed, so the theory goes, it just changes from one form into another.

When you run a petrol engine, it consumes fuel which contains potential energy PE. Let's say a unit of fuel contains 100 joules of energy.

The engine extracts this energy from the fuel and converts it from PE into other forms of energy. Typical figures for this will be to provide 30 joules of kinetic energy (KE) for the car, and 70 joules of energy will end up as heat which is dissipated away through the radiator / block / etc. The 30 + 70 adds up to the 100 joules of PE you started with, and no energy is created or destroyed. The 30 joule against 70 joule figure is what provides the thermal efficiency value, i.e. 30% efficient in this case.

What happens to the 30 joules of KE afterwards is irrelevant, it's not a property of the engine at that point. It could be used to winch a heavy weight up a mine shaft, in which case it basically ends up as PE, not as wasted heat energy.

Note specifically that in the same way a diesel engine works at 40% thermal efficiency, i.e. it delivers 40 joules of mechanical energy against 60 joules of heat.

If all the energy of the fuel, as Steve "Pointless" Firth suggests, turned into dissipated heat, then 100 joules - 100 joules of heat would leave 0 joules of mechanical energy to use in driving the car, and this would be a pointless engine, running on the Steve Firth's Posts principle. You would want to use an engine like that about as much as you'd learn something from Steve "Pointless" Firth.

Of course, some of the energy will end up being used to make sound, some will do work wearing out the engine, etc, etc, but the bulk of the energy is either motive power or thermal waste and it's good enough to ignore those other losses for the purposes of comparing petrol against diesel.

It is a simple concept and really I'd expect you to understand it.

My guess would be an F1 car cylinder is approx 300cc...

i thought they were 2500cc? (whole engine I mean)

Nope. 3l at the moment, with proposals to go to 2.4l V8 at some point. The idea being to cut costs and make development easier by lopping off a couple of cylinders. I'm not exactly convinced that'll save any money whatsoever, and IMO it'll cost more in terms of development for obvious reasons. Plus the teams spend what they've got, so if they're spending say 50 million a year right now on engine programs, they'll be spending 50 million a year on V8 engines if they come in.

I think the actual figures for F1 engines when you can get your hands on them (very few specs are ever released) is somewhere between 2997 and

Drive around then return to where you started from and stop, what happened to the energy in the fuel?

Yes that was a stupid rule just like the V8 rule will be. It IIRC was also a cost cutting idea but all it did was limit the possibilities for the engine design or we could be seeing V8s,10s and 12s on the grid now maybe even a few horizontally opposed engines. The FIA should have set a maximum displacement then left it at that.

.0V6 include Rover - 150bhp Nissan QX - 140bhp Mazda Xedos -145bhp Maserati Ghibli - 330bhp Mitsubishi FTO - 200bhp

Can't think of any others - jag is a 2.1 isn't it?

shouldn't make any difference if its a straight 6 or a V6 really, or even a flat 6. It's the number of cylinders, the amount of valve area etc.

Truth of the matter is a 6 will always be smoother than a 4 due to more even firing impulses

some more 2.0 6 pots include

Lexus IS200 BMW 320 (when it WAS a 2.0) Mazda 323F 2.0

none of which are particularly pokey.

the Italians have a habit of weird 2.0s.. Ferrari 208 for example.