Originally Posted By: Langanobob
Yeah but...the pressure on the combustion chamber side of the valves is never zero so to some extent there should be a compensating effect. That seems to be the opinion of automotive engineers who don't subscribe to turbo pressure lifting valves.
Let's think about the 4-stroke cycle, and when the intake valve(s) could be blown off their seat(s).
1. During the power stroke, cylinder pressure is obviously much higher than intake manifold pressure, peaking at ~2400 psi for a fairly modern diesel engine, and decreasing to ~180 psi at exhaust valve opening. So there is no way the intakes will be blown open.
2. During the exhaust stroke, after cylinder pressure has blown down to match exhaust manifold pressure, is the most likely time the intakes can be blown open. If exhaust manifold pressure is equal to intake manifold pressure, there is no net pressure force acting on the valve, so it remains seated with the net force of the valve spring closed load. In most turbodiesels, exhaust manifold pressure will be higher than intake manifold pressure due to the small turbine housing inlet area. But I expect in high performance applications where turbo lag is tolerated, intake manifold pressure can be higher than exhaust manifold pressure, so there may be a net pressure tending to blow open, but this would only be on the order of ~10 psi (as a guess).
3. During the intake stroke, the valves are open, so the issue is moot.
4. During the compression stroke, the valve closes, trapping the fresh charge air at some pressure close to intake manifold pressure. Then the ascending piston causes cylinder pressure to rise above intake manifold pressure, making it impossible for the valves to blow open.
On the other side of engine breathing, it is a real concern for exhaust valves blowing open in applications where exhaust brakes are used. Cummins used to have different exhaust valve springs for those applications, that had high seat loads. I recollect that 60 psi was the design pressure for those springs.