Why r big engine cars so inefficient @driving around slowly?

[Mr2Mike]

Mave said:

But for a given power, isn't the CR of a turbocharged car the same as an NA car of the same capacity?

No, since as already stated, to get the same CR you need to have more air and fuel in the cylinder, and hence more power.

Mave said:

The restriction of the turbo operating off boost does the same job as the throttle in limiting air flow. Where do the higher pumping losses come from?

Pumping losses don't only apply to the inlet, any restriction in the exhaust (such as e.g. a turbocharger) increases pumping losses.

[Mave]

Mr2Mike said:

Mave said:

But for a given power, isn't the CR of a turbocharged car the same as an NA car of the same capacity?

No, since as already stated, to get the same CR you need to have more air and fuel in the cylinder, and hence more power.

Mave said:

The restriction of the turbo operating off boost does the same job as the throttle in limiting air flow. Where do the higher pumping losses come from?

Pumping losses don't only apply to the inlet, any restriction in the exhaust (such as e.g. a turbocharger) increases pumping losses.

OK, bear with me....
The effective compression ratio isn't just what the piston does to the air; it's the whole cycle up to TDC, you take air at atmospheric pressure, reduce it through a throttle, and compress it with the pistons. If you've got a turbocharger, then that may increase or decrease the pressure as well.

For a given capacity, pressure and CR are directly linked, all you are doing is using the tubocharger to increase or decrease air flow, and hence CR, in the same way as a throttle plate. It's just you can increase as well as decrease the CR. Or am I missing something? (I know that temperature is inportant, I'm trying not to get hung up on the effect of intercoolers).

Regarding the pumping losses, the turbo certainly introduces those, but these are low when the turbo isn't doing any work, and the pumpin glosses on teh exhaust stroke are lower as you aren't using the exhaust gasses to drive such a high CR.....