On the Diesel V Petrol debate...

Cool :)

Only thing that ever worried me about them was they look pretty close to the water - What happens if you want to cross, say, the North Sea in one? I can't see how they'd ride anything but small waves very well.

Otherwise, a very good idea that really ought to be looked into more, especially for cheap ferry crossing journeys, IMO.

The point is that all the energy released goes to heating things up sooner or later.

If the fuel/air mixture were enclosed in a fixed volume then yes, he would be correct, but we are talking about an engine, and that has pistons, that can move.

It is normal to include the piston when analysing the engine, just google for "otto cycle". In terms of the power stroke, energy is released by the fuel/air mixture and heat energy and work is taken out. You can see from a PV diagram how much work is generated. You can then calculate the efficiency from the net useful work produced over the whole 4-stroke cycle.

Read the original post by "Sales!", we're talking about an engine.

Steve pointed out that all there energy is eventually released as heat it may go through locomotive energy in pistons to drive the car but what happens to that energy when you stop? (As you can see we are both arguing from completely different fundamentals that aren't mutually exclusive.)

This one can manage one foot high waves, or something, so no, it's useless in anything other than a very calm sea.

The bigger versions can probably handle bigger waves, but it's a weakness in the design. The Russians had it for carrying tanks / troops rapidly across the black sea, where waves aren't much of a problem.

There are some practical limitations that make it not as excellent as all that, I reckon. E.g. commuting to London on the Thames, you'd have to dodge bridges and other boats, locks would be a problem, etc. You can't slow down from 90 mph or you land in the water, and have trouble taking off again, etc. 20 feet wide is more than most river craft and too wide for a lock, etc. You could shoot the weir, but not climb back up on the way home.

Maybe. They could certainly look into this with someone like P&O, although I suspect the channel is too rough most of the time.

As for a personal cross channel ferry, the sea isn't ok, it's too rough almost all the time. You can get a real boat, that's not especially trivial to sail across the channel either, but can do it in worse weather than a ground effect plane, I would say...

Well duh of course it all ends up as heat *eventually* (unless you go up a big hill), but that's not really relevent is it? We're talking about an

*engine*, that produces useful work for us. Of course that useful work will eventually be turned into heat, but that's not the point.

Again a bit pointless, but you will have got some "useful" work out of it, even if it is only to power the dyno and generate heat.

Indeed all the fuel will end up as heat, but some of it will be converted straight to heat, while the rest of the energy will be converted to *useful work* and then to heat at a later stage.

Do you have a learning disability? I've said quite the opposite.

OK OK, fair enough, there's no point in arguing, we all seem to know what we're talking about :-)

It's just to measure the efficiency you need to be able to measure the locomotive energy before it goes back to heat again (whether that be through a dyno, through brake pads, etc etc).

Not quite, because the headline power figure may only be between 5999 rpm and 6000 rpm and it might drop off very quickly either side of that range.

Given that you don't have a continuous variable transmission you need to take into account how the power varies across the range of rpms that you are going to be using.

Well it passed some time on a Friday afternoon. ;)

Yeah but it's just another pointless complaint from Steve "pointless" Firth. Any search on usenet will produce the 30/40% thermal efficiency figure without any real effort on the surfer's part.

So let's have a poll:

(1) Diesel engines convert their fuel energy into roughly 33% more work rather than heat, compared to petrol ones. (2) Both engines dissipate all their fuel energy as heat.

I vote (1)

The only reason Steve "pointless" Firth is pushing this, is a pointless attempt to make someone else appear to have made a wrong statement. He's not actually as stupid as he tries to appear by misunderstanding simple statements, he is just being malicious and trying to make them mean something different. He knows perfectly well there was nothing wrong with the original statement.

Steve "pointless" Firth does this for reasons of his personality, it's not about constructive research or learning stuff. The syndrome can be terminal, e.g. he hasn't got outlets for his behaviour in real life any more, but usenet can't politely make excuses and leave, or punch him on the nose. I'll make a prediction that he continues to behave like this for years and years to come, as it is a compulsive disorder and he won't break the cycle. See if I'm right on that one as well. I won't see, as I have filtered him.

You've hit the nail on the head their matey. Unfortunately I lost my killfile when I got a new PC, but am rebuilding it steadily... :-)

And got me thinking about thermo again, not sure whether that's a good thing or not!

Exactly, you need to calculate the input energy (from the calorific value of the fuel) and measure the work done by the engine (on a dyno or WHY). You cannot calculate the thermal efficiency of an engine by measuring the work output and heat output of the engine because the heat is a combination of the heat losses and the heat that results from the work being done by the engine and it's damn difficult to separate the two.

Yes and anyone with more than half a brain could tell he was wrong. He doesn't like being wrong so he's had a pout and a sulk, looks like you're going to join him, diddums.

Indeed - all the energy in the fuel is converted to heat - the air expands and the fuels become gaseous and the expansion forces the piston down.

Ultimately, power output is about the amount of air you can get through the engine in the least time, so bigger valves (4 per cyl, large bore so short stroke, high revving motor) and forced induction are they keys. Diesels will always be down on power until we get a 9000 rpm 32 valve V8.

A 2l straight six has probably more power potential than a 2l four because it is likely to have more valve area and probably a shorter stroke.

Indeed no-one said anything about thermal efficiency, the energy in the fuel is released as heat, which causes expansion of the cylinder contents and moves the piston. Some of the heat is wasted heating the components around it, hence maybe 70% is lost in just heating things other than the cylinder contents, hence this example is 30% efficient.

Err you're not right there, from: Message-ID:

"A diesel engine, all else being equal, will have a thermal efficiency around 40%, while an otto cycle petrol engine will manage around 30%, "

So someone most emphatically did say something about thermal efficiency, then they ballsed up the definition of thermal efficiency which was what I commented on. The fact that "thicky" Sales! was having a go at how wonderful he was and how stoopid he saw me as being was most amusing from my PoV after his schoolboy howler.