Scavenging vs non scavenging
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BEATAV8
The Engine Whisperer
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- Feb 14, 2002
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Scavenging is a thing for naturally aspirated engines.
The manifolds on a turbo car are highly pressurized so there’s no considerations for scavenging
The manifolds on a turbo car are highly pressurized so there’s no considerations for scavenging
BEATAV8 hit the nail on the head, but this could be a fun topic for discussion. On small turbos like our cars run the exhaust pressure vs intake pressure is high enough that you are not going to get any scavenging in the sense of a naturally aspirated engine. If we had longer tubes on the exhaust, could we get a marginally lower cylinder pressure via the exhaust pulse, maybe, but the intake charge is still going to be diluted because you are not going to drop the cylinder pressure lower than the intake pressure like what happens on an NA application.
Now, if (a big if) the newer turbo technology is allowing for much better exhaust pressure vs intake pressure, a long tube header could be a benefit. A balance between spool and efficiency has to be managed. It is possible to have the exhaust pressure equal to or lower than intake pressure, but historically that has resulted in a spool up time that is not useful except for a purpose built machine.
Depending on that pressure ratio, the header could be optimized along with cam timing. If you could actually have a turbo designed to run at a lower exhaust pressure than boost pressure (and still have good spool response), the wasted exhaust energy could be recovered as a pumping gain because opening the intake valve would pressurize the cylinder above exhaust pressure and recover some of the pumping losses.
Consider as rough figure as I don't have the heating values for any fuels at my finger tips along with the efficiency figures for typical turbo engine, but roughly 2/3rds of the HP available in the fuel goes out the tail pipe and the radiator. Think about that. A turbo running an exhaust pressure less that the intake pressure can improve that figure.
Now, if (a big if) the newer turbo technology is allowing for much better exhaust pressure vs intake pressure, a long tube header could be a benefit. A balance between spool and efficiency has to be managed. It is possible to have the exhaust pressure equal to or lower than intake pressure, but historically that has resulted in a spool up time that is not useful except for a purpose built machine.
Depending on that pressure ratio, the header could be optimized along with cam timing. If you could actually have a turbo designed to run at a lower exhaust pressure than boost pressure (and still have good spool response), the wasted exhaust energy could be recovered as a pumping gain because opening the intake valve would pressurize the cylinder above exhaust pressure and recover some of the pumping losses.
Consider as rough figure as I don't have the heating values for any fuels at my finger tips along with the efficiency figures for typical turbo engine, but roughly 2/3rds of the HP available in the fuel goes out the tail pipe and the radiator. Think about that. A turbo running an exhaust pressure less that the intake pressure can improve that figure.
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