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Old 05-17-2012, 09:28 AM   #28
RickB
Scraping Paint
 
City: Fort Lauderdale
Vessel Model: CHB 48 Zodiac YL 4.2
Join Date: Oct 2007
Posts: 3,804
Quote:
Originally Posted by manyboats View Post
2 strokes over scavenge.
So do 4 strokes, particularly supercharged 4 strokes. Valve overlap allows complete scavenging and cools exhaust valves which improves engine life.


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That is they exhaust more than their exhaust and tend to leave less air in the chamber when the compression stroke begins. This leaves less air in chamber to compress so less pressure is present at TDC. Also this extra air cools the cylinder causing thermal loss and less efficiency.
No, the exhaust valves close first and that is where the “super” in supercharging comes from. The cylinder pressure goes above atmospheric.

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If the blower pumps more air into the chamber than it can use it's energy lost. And if the blower pumps less than is required or needed less pressure will result.
See the comment above. It is not ‘energy lost” it is energy expended to clear the cylinder of exhaust gases and thereby increase the power available and the overall efficiency of the cycle. That "extra" air permits more fuel to be burned than if the engine relied on any other type of scavenging, and more fuel burned equals more power.

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Everything is not always right in a 2 stroke. There is always some hot oxygen starved air remaining in the cylinder when the compression starts.
Virtually none, and far less than a normally aspirated 4 stroke.


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And the blower is required to pump a bit of excess air out into the exhaust ...... a waste of energy.
See my first comment. Extending engine life and increasing efficiency is not a waste of anything.

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Lats but by no means least is that the plower takes gobs of power to run.
That is the “cost of doing business.” A fuel injection pump and a coolant pump does too but the overall benefits provide very high returns. A mechanical surpercharger provides benefits not even mentioned in this thread. The performance and history of the DD 2-strokes is witness to the fact that the benefits far outweigh what some people mistakenly believe are detriments.

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The above applies only to DD type diesels w poppet exhaust valves.
Which perform cylinder scavenging exactly the same as large slow-speed 2 stroke diesels which achieve thermal efficiency in the low to mid 50 percent range at full power. The only difference is the large slow-speed engines use turbochargers and have a single very large exhaust valve.



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