Turbochargers Anyone?, General question about turbochargers

[combwork]

Hi. There's s question I've asked a number of people; the answers always been the same "Dunno Mate". As I understand it, turbocharged engines have a lower compression ratio than non turbocharged engines. This I am told is so that when the charger kicks in, the extra volume of fuel/air mix does not cause such an increase in cylinder pressure as to damage the engine.

Question. Why not keep to standard (non turbocharged) compression ratio, but reduce the output of the turbocharger so that when operating, fuel/air volume is the same as it would be had the compression ratio been reduced, but the turbocharger output increased?

This would mean that with a clutch between the driven (exhaust) and driving (induction) fans, with clutch disengaged the car would drive like any other non turbocharged car with same size engine; both fans freewheeling. Engage the clutch and with the fans locked together, you get full power turbo performance, with corressponding 'full power' fuel bills. Engage/disingage could be done electromechanically with a dashboard switch in much the same way as the compressor on an aircon system is engaged and disengaged via a clutch.

To avoid any kind of drag that the freewheeling induction fan might present to the engine, a belt drive could be fitted between induction fan and camshaft so that the disengaged fan always spun at engine speed; not fast enough to increse induction flow; not slow enough to impeed it. The belt drive could operate via the same clutch that engages the turbocharger. Turbo on, belt drive off; turbo off, belt drive on. There could be a problem here with any drag presented by the freewheeling induction fan only being compensated for after the engine speed has increased, so a possible alternative might be to spin the freewheeling fan via an electric motor linked to the throttle potentiometer. This could be set so the fan didn't work like an electric supercharger; the electric motor would be there simply to overcome drag or inertia presented by the induction fan. The motor would switch off the moment the clutch between driven and driving fans engaged.

Is this possible, practical or pointless......................?

Jim.

This post has been edited by combwork on January 04, 2009 12:04 pm

[Peter.N.]

Hi Jim

The compression ratio is lower to reduce the stress on the engine with increased charge pressure, as I am sure you appreciate, but thats not the only difference, the pistons and con rods are strengthened and the pistons oil cooled.

The point of a turbocharger is not just to increase the power, it also makes the engine more efficient because at anything less than full throttle its running on a weaker mixture than its normally aspirated counterpart.

Your suggested 'modification' may well work, but its a lot cheaper to just use slightly shorter pistons and any loss of efficiency due to the lower compression ratio is more than made up by the increase of efficiency due to the torbo.

To sum up: If you have a turbo fitted it pays to get the full benifit from it all the time.

Hope that makes sense

Peter.N.

[DoubleChevron]

Lower compression ratio == more room for fuel and air

[terry g]

compression ratio, turbo/non turbo the diffrence is to stop detonation on petrol engines, , would it not be simpler to bypass the inlet / air filter pipe then use a butterfy vave to switch between the two, also linked to the waste gate?

[Peter.N.]

I don't know anything about petrol engines since they did away with the carburetter and distributer but the lower compression ratio is as you say to prevent detonation, on a diesel engine you always get detonation - that's how they work, but they still have a lower compression ratio.

I think manufcturers will always do things the simplest way as it makes for reliability - except with the electronics of course, but I think that has probably been foisted on them as the only way to get emissions down.

Peter.N.