Nomad Willy
Guru
I've been meaning to start a conversation highly related to what is being discussed on the twins thread.
When a diesel is at WOT propped correctly it makes it's maximum hp and I assume it's fuel injection system is, at that point injecting the optimal amount of fuel. But that is only one point of operation and one that most of us only use or attain for brief periods so the fact (assumed) that the mixture is perfect there is not very meaningful to anyone here. The mixture at lower engine loads where we run our engines could be very important to everyone.
When we look at the load curve and the power curve a great difference will be apparent. Most power curves are convex. That is the power rises steeply at half engine speed and near the top end flattens out in a shape that could be considered a hump. The resistance of a FD hull is a concave curve. Rising slowly at first and then more and finally rising steeply to hull speed and a bit beyond.
So backing off from WOT very quickly the resistance drops dramatically but the power available remains close to maximum. Suddenly there's a great difference between required engine output and power available.
On an old fashioned mechanically injected engine the amount of fuel injected is controlled by the fuel injection control rod and I think that at 50% deflection of the arm connected to the control rod is half way between idle fuel amount and WOT fuel amount. So roughly half as much fuel is injected at half "throttle". It would seem to me that if I was at half throttle I would NEED to be at half rpm to have a proper fuel mixture. And the amount of fuel burned is proportional to the power output and way less than half power is at half throttle. Too many variables to have that just happen to come about and at many other engine speeds that aren't rising and falling exactly together there should be a lot of mismatches in load, speed and throttle position. Yet the mechanically injected engines only smoke from too much fuel on rare occasions. And that's just considering drastically too rich of a mixture. Perhaps a mixture of too lean is much more of a problem. Does the load, engine speed and injected amount of fuel mostly come out right across the span of loads and speeds? Dosn't seem likely to me.
Then there is the modern computer controlled engines and I suspect they have sensors to monitor the load, temps, and anything else pertinent and then adjust the injection to inject the perfect amount of fuel for any anticipated situation.
Well it looks to me like the computer controlled engine should be far better than their reputation and I'm surprised they are only 10 or 15% more efficient. And there should be lots of other benefits like far less fuel oil contamination of lube oil from rich mixtures and excess fuel leaking down the cylinder walls and into the lube oil.
But more important to me and probably most here is how far astray does the mixture get in actual mechanical engines propped correctly. I'm sure over propped engines run too rich. Will under propping result in better mixture control than correctly propped engines? I was under propped for a time in Alaska by 100rpm and the engine seemed to run better than otherwise but that's very subjective.
What can the most knowledgeable tell us here about our fuel mixture in our typical dynamic conditions.
I'd like to understand this better.
When a diesel is at WOT propped correctly it makes it's maximum hp and I assume it's fuel injection system is, at that point injecting the optimal amount of fuel. But that is only one point of operation and one that most of us only use or attain for brief periods so the fact (assumed) that the mixture is perfect there is not very meaningful to anyone here. The mixture at lower engine loads where we run our engines could be very important to everyone.
When we look at the load curve and the power curve a great difference will be apparent. Most power curves are convex. That is the power rises steeply at half engine speed and near the top end flattens out in a shape that could be considered a hump. The resistance of a FD hull is a concave curve. Rising slowly at first and then more and finally rising steeply to hull speed and a bit beyond.
So backing off from WOT very quickly the resistance drops dramatically but the power available remains close to maximum. Suddenly there's a great difference between required engine output and power available.
On an old fashioned mechanically injected engine the amount of fuel injected is controlled by the fuel injection control rod and I think that at 50% deflection of the arm connected to the control rod is half way between idle fuel amount and WOT fuel amount. So roughly half as much fuel is injected at half "throttle". It would seem to me that if I was at half throttle I would NEED to be at half rpm to have a proper fuel mixture. And the amount of fuel burned is proportional to the power output and way less than half power is at half throttle. Too many variables to have that just happen to come about and at many other engine speeds that aren't rising and falling exactly together there should be a lot of mismatches in load, speed and throttle position. Yet the mechanically injected engines only smoke from too much fuel on rare occasions. And that's just considering drastically too rich of a mixture. Perhaps a mixture of too lean is much more of a problem. Does the load, engine speed and injected amount of fuel mostly come out right across the span of loads and speeds? Dosn't seem likely to me.
Then there is the modern computer controlled engines and I suspect they have sensors to monitor the load, temps, and anything else pertinent and then adjust the injection to inject the perfect amount of fuel for any anticipated situation.
Well it looks to me like the computer controlled engine should be far better than their reputation and I'm surprised they are only 10 or 15% more efficient. And there should be lots of other benefits like far less fuel oil contamination of lube oil from rich mixtures and excess fuel leaking down the cylinder walls and into the lube oil.
But more important to me and probably most here is how far astray does the mixture get in actual mechanical engines propped correctly. I'm sure over propped engines run too rich. Will under propping result in better mixture control than correctly propped engines? I was under propped for a time in Alaska by 100rpm and the engine seemed to run better than otherwise but that's very subjective.
What can the most knowledgeable tell us here about our fuel mixture in our typical dynamic conditions.
I'd like to understand this better.