Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

While not perfect the ai helps

* **Incomplete exhaust:**
I think the ai text is stating that the port at bdc must take away from the total stroke. If it's open at the last 1% then that engine is only 99% as efficient as one that doesn't have a port.

Also that port stops the scavenging effect of the valved exhaust. Although perhaps that could be mitigated by routing both to a common exhaust pipe.

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* **Fresh air loss:**
Indeed it was clumsy on its wording for the intake stroke. Basically on the bdc of the intake/compression stroke some of the intake charge will be lost to the open exhaust port.

Good points. 

However regarding exhaust port taking away a bit of the power stroke, there likely would not be a noticeable efficiency or power loss given that the the bottom 5-10% of the power stroke provides a miniscule amount of power/torque given the limited amount of piston travel. This open port design clearly did not grow much over time with the exception being 2 cycle engines. As for scavenging, the intake valve on a Gade is atmospheric and doesn't let in any fuel/air until the piston is traveling downward and creates a vacuum. On non-hit & miss engines (without any open ports) the intake cam lobe could be timed to begin opening before top dead center on the exhaust stroke which allows for some scavenging.