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Does anyone know why the mean piston speed might be lower in the average turbo gasoline engine compared to the average naturally aspirated engine?

Scratching my head to figure this out but no luck so far. Any advice would be appreciated!

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1 Answer 1

tl;dr: It depends.

Quoting this (admittedly incomplete) wikipedia page:

The mean piston speed is the average speed of the piston in a reciprocating engine. It is a function of stroke and RPM. There is a factor of 2 in the equation to account for one stroke to occur in 1/2 of a crank revolution (or alternatively: two strokes per one crank revolution) and a '60' to convert seconds from minutes in the RPM term.

MPS = 2 * Stroke * RPM / 60

For example, a piston in an automobile engine which has a stroke of 90 mm will have a mean speed at 3000 rpm of

2 * (90 / 1000) * 3000 / 60 = 9 m/s.

So, if what you're describing were the case (that turbo engines tend to have lower mean piston speeds), we'd expect to see a trend towards either low stroke, low RPMs or both.

For a turbo engine, we care a lot about airflow. The airflow rate is proportional to the displacement times the engine RPM times volumetric efficiency (see Maximum Boost page 27 for the real equation). Our turbos generate more boost as RPMs increase and then being to drop off as efficiency goes down. So I wouldn't expect to see a bias towards low RPMs on a turbo car.

However, we might expect to see turbo engines with smaller stroke and larger bore for a given displacement. In the absence of detailed information, you might expect a short stroke engine to be high rev-er since each revolution would require a shorter transit for each piston (i.e., it doesn't have to go as far so it's likely able to get from top to bottom more quickly).

So, that tells us that a given turbo engine might have high revs and short stroke. Does that mean that we'll see higher mean piston speeds? As always, it depends on the specific engine and application.

When I'm driving my son to school in my turbo car, the revs are pretty sedate. I would expect to see pretty low mean piston speeds. This same car is used for rallying where they're using the top end of the rev range quite a bit more. I'd say that their mean piston speeds are pretty spritely.

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