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Technical engine questions


Andrew
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Woppum should like this one.

Chap at my work in one of my teams rebuilds engines all the time and also writes his own management code. His speciality seems to be fitting rather large powerful engines to Clios (that eat E92 M3s he assures me - no reason to doubt it!). That said, I think his last massive effort was a Calibra engine in a Nova. Anyways, I was in the work kitchen chatting to him and he told me a few things about engines I'd never have known; like how one of the piston rings is a cleaner to rid the cylinder of oil.

So, this thread is about asking bone questions about engines. Sure, you could Google it, but that doesn't often throw up as much interesting stuff as threads on here. Also feel free to post something interesting about engines - you might like to tell us how double VANOS works for instance.

So, my first question, a turbo, what gives? I get it runs off exhaust gasses but what does the pressure of exhaust gas do to help the power?

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Turbo - spins at variable speeds. Those speeds are governed by the 'speed' of the exhaust gas. The faster the exhaust gas, the faster the turbo spins and as a result it pushes more air into the engine, as it usually has one shaft connecting the two turbines. (one turbine for exhaust, one for input). This in turn makes the engine revs rise, which produces more exhaust gas which spins the turbo faster...... And so the cycle continues.

I think.

I also think a turbo gives an additional 20-30% more power than a n/a engine of the same size? (or used to, back in the day!)

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The exhaust gas is used to drive a turbine, turning some of the kinetic energy of the gas into rotation of the turbine shaft. On the other end of the shaft is a turbine that blows air into the inlet. So, in effect, the engine is fed with compressed air instead of atmospheric pressure air, meaning more oxygen gets into the cylinder. More oxygen means you can put more fuel in, hence a bigger bang.

Or, as Clarkson put it, exhaust gases go into the turbo,... umm... witchcraft happens, and you go faster +++

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Turbo's have a spinny thing, that's spinned by the engine farts - the spinny thing squishes more air into the bang chambers that are allowed to get extra gogo juice to make the bigger bang noises to make the more umph and vrooom, wooosh, vrooooom, whoooosh :grin:

Sorry, yes. I meant to say "Spinny thing make go faster. Wheeeee!" :grin:

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Ok, an ideal explanation, thanks. Who wants to tackle variable vane technology (he says, reaching into the back of his mind at a Porsche Turbo article in a year's old magazine)?

ah yes, variable vane technology - based on the humble weather vane, when attached to the roof of the Porsche it allows different levels of boost depending on the direction the wind is blowing +++

WeatherVane.jpg

....I'm sure this is what Andrew had in mind?

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Who wants to tackle variable vane technology

Oh Lordy.

Ummmm.

This might be utter b0llocks, but here goes.

The turbine bit is basically a windmill. Blades stuck in the airflow are spun by the air passing over them. Or vice-versa, the spinning blades cause the air to move. If you look at the blades in a turbo, and compare them to those in a gas turbine engine (jet engine...) or to a wind turbine, they all look different. This is because each is designed to operate at peak efficiency in the intended context - which dictates a different shape dependent on the temperature, pressure, airspeed, size and so on.

So variable vane technology adjusts the vane angles of the turbine according to the current engine speed, air pressure, and engine demands so as to tune the turbine and get best efficiency out of it.

I think.

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This also might be rubbish but I think in the 'old days' to prevent lag you might have a setup with two turbo's - one low pressure, one high pressure - at a guess variable vane gets rid of that by allowing the vanes to change angle so one monster dustbin lid size turbo works well at low engine speeds and high engine speeds...

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Cool, I get that. Nice one. I might have my membership recinded on here by a mod given my apparent lack of engine knowledge so I'll bang another one out quickly!

What is it that limits an engine speed? I'd imagine in a diesel it is about the speed the fuel burns however why is it in the new Ferrari you can rev to 9000rpm on the same fuel as another supercar which might top out at 7500? Plus of course you have motorbikes which rev out to 12k, but I think they have different valve setups so maybe they don't count. Is the speed perhaps limited by the weight of the piston the engine is chucking about - but if that is the case then surely most manufacturers can get fairly close to each other in that respect.

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What limits engine speed - basically how fast you can fill a cylinder with air, fuel, make a bang, get rid of exhaust and refill it! When that all work perfectly then piston acceleration limits and using pneumatic valves keep increasing rpm and friction/inertia play huge roles. At the simplest level, for racing cars, expensive engines rev higher!

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Motorbikes rev way past 12k these day, especially the smaller capacities. I believe the limitation is the reciprocating mass of the piston. If you look at the piston on a modern bike, it's more like a disc than a traditional cylinder shape.

I would also expect valve operation to be a constraint too.

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They consume less petrol however the latest wisdom is that you save even more when you disengage the transmission.

The latest VW/Audi range vehicles have a coast mode that is said to save even more fuel than leaving the transmission engaged, as with no or should I say less friction than keeping the car in gear. This is now found to be better than leaving it in gear apparently.

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