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Exhaust valve opening time

Engine Building Fundamentals

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Hi Andre,

I was just wondering, would there be gains having the exhaust camshaft open the exhaust valves prior to reaching BDC on the power stroke and continuing its duration as per normal.

With optimum power being around 16° ATDC, would having the exhaust valves begin opening at around 100° ATDC (or earlier/later) and fully open by 180° BDC or earlier. This allowing the crank to relatively free spin.

In turbo applications further combustion after its initial power position of around 16° allowed to escape and possibly help spool a turbo.


Hi Terry depends fully on the engine type and application but an exhaust valve generally opens @ 50-60 bbdc ad is closed around 0 deg, opening the valve too early will no be beneficial to complete combustion on the power stroke,

it also can get very close to pistons in many cases so that is another caution to be aware of

regards Ross

Exhaust will normally close some degrees after TDC, sometimes a surprising amount - but that's more N/A than for exhaust driven superchargers.

As Ross said, you do have to be careful as the piston is chasing the closing exhaust valve, and any weaknesses in the valve train, especially valve springs, could allow it to catch up - with noisy and expensive results.

The only time when opening exhaust valve early in the cycle can be beneficial is with very high CR when the most of the work is done while piston is not too far away from TDC, so there is very little sense to keep the valve closed and exhaust camshaft with longer duration can help better scavenging thus making more power. Usually you want that combination with high CR - bigger intake valve and longer duration of exhaust camshaft.

Hi guys,

thanks for the responses, I seethe thought i had is not entirely impractical. If only i could find the virtual engine building program i used to have, i would play with the random ideas on there and watch the theoretical results.

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