What you need to know about Brake Specific Fuel Consumption

[drmiller100]

e40, i thought you wanted some ideas on how to go about getting an engine to run in lean burn.

your boat is a BAD place for lean burn - too much load. And idle won't work either.

if you apply your basic approach and drop the mixture to 18 to 20 to one, watching your egt's, you will be there.

but 14 is burn down - so don't ease into it.

you might reconsider your answer on the EGR.

[E4ODnut]

On any of my engines I've found that ~ 18:1 is about as lean as I can go without a lean stumble. 17:1 seems to be sustainable, but only under quite light loads.

EGTs don't mean much because of the water cooled manifolds. The highest I've recorded is ~ 650 degrees F.

Engine loading on a boat depends on where the shaft speed is in relation to the propeller power demand curve. My normal cruise is ~ 2000 RPM, ~60 KPA manifold absolute pressure (~ 12.5" HG manifold vacuum) load. I consider that to be a relatively light load.

Why would I want to consider EGR?

[George Tyler]

Originally Posted by drmiller100

so.........

if we could figure out how to run a gasoline engine without a throttle plate, our CITY mileage would improve 20 to 30 percent.

probably not as much gain on the highway.

there is a hydrogen powered V8 with no throttles, hydrogen can burn with a 5% ratio and power was controlled just be changing the amount of hydrogen injected. I found it with google, but can't see it quickly now.

[serialk11r]

Originally Posted by drmiller100

so.........

if we could figure out how to run a gasoline engine without a throttle plate, our CITY mileage would improve 20 to 30 percent.

probably not as much gain on the highway.

Well it'd save you at idle but when accelerating the throttle doesn't consume much power, so I dunno, depends a lot on driving style (engine off coast hypermilers would see little benefit) and what you call "city" conditions. In the future I think we'll start to see manufacturers have the throttle wide open when engine braking, as soon as regenerative brakes of some sort become standard.

On the highway it should be worth a few percent, since you're always holding part load.

[user removed]

I have read posts from a few people who think having the throttle open on deceleration would reduce engine drag. I think it might actually increase engine drag. Let me explain my thinking.

When you have 20 inches of manifold vacuum you are only allowing about 1/3 of atmospheric pressure into the cylinders. Lets say 5 PSI instead of 14.7 PSI.

You compress that 5 PSI to 1/10 of that volume for a total of 50 PSI pressure on the piston surface per square inch of surface area.

Now if you keep the throttle at WOT then you are compressing 14.7 PSI atmospheric to 10 times that amount or 147 PSI of pressure per square inch.

Compared to 50 PSI that's significantly more energy required to compress the air. Now it is understood that that same pressure will help to push the piston down after TDC, but with the engine already pulling the piston down I would tend to believe it would take more work to move 3 times the amount air through the engine.

Another reason I think this is true is when you check the compression of any engine, you have to open the throttle to get a better reading. When you do this the cranking speed slows down slightly, which also indicates it takes more work to push the air through the engine with the throttle wide open.

Not meant to be any argument but the slower cranking during a compression test, with the throttle open, is fairly good evidence.

regards
Mech

[E4ODnut]

Old Mech,
If I understand your post correctly you are talking about engine braking, for instance, descending a hill, throttle closed. The theory is that if you open the throttle, engine braking would be reduced?

I agree with that. No theory, just observation. When I am descending a grade, throttle closed, I can shut off my fuel injectors. Minimum manifold pressure (max vacuum), no power developed in the engine. If I open the throttle the injectors will fire again, but, I can use manual control of my idle air valve to imitate a partial throttle opening. If I do this, manifold pressure increases (decrease in vacuum), still no power being produced, but the vehicle speeds up.

[serialk11r]

Yea I've thought of that too, but apparently BMW's newer cars have reduced engine braking, and some article I read said it was because it opened the throttle. I don't know how accurate this is though. It's possible that they deactivate the valves completely.

When opening the throttle all the way, most of the compression energy is returned to the piston but not all because exhaust valves open early (of course this is dependent on intake timing too). If the intake were completely plugged and no air were allowed to flow into the engine, the engine would expend energy expanding air at some equilibrium pressure in the manifold, and when the exhaust opens it would allow air to go into the cylinder, and then be pushed back out, not recovering energy. If a little bit of air were allowed to flow through, the energy lost at expansion decreases, but the energy lost to the throttle goes up. So I think it's hard to say...your cranking anecdote suggests it may be at some intermediate throttle opening.

[mort]

Originally Posted by Old Mechanic

Another reason I think this is true is when you check the compression of any engine, you have to open the throttle to get a better reading. When you do this the cranking speed slows down slightly, which also indicates it takes more work to push the air through the engine with the throttle wide open.
regards
Mech

Hi Mech,
Is open throttle pumping through the exhaust valve (etc.) a bigger load than closed throttle pumping through the throttle (etc.)? A real time measurement of cylinder pressure during a cranking compression test might tell - maybe somebody could check it out?

-mort

[drmiller100]

Originally Posted by E4ODnut

On any of my engines I've found that ~ 18:1 is about as lean as I can go without a lean stumble. 17:1 seems to be sustainable, but only under quite light loads.



Why would I want to consider EGR?

advance your timing for lean burn, and increase throttle position a touch. this won't work very well on a boat - too much load.

[George Tyler]

I have done a test, running down a hill, turned the key off (and locked the steering! not reccomented). there is quite a difference in deceleration with open and closed throttle, more breaking with closed throttle. I also did it with a 2 stroke bike, it behaves exactly the opporsite! which makes me think it may be possible to make a 2 stroke more efficient at low throttle openings than a 4 stroke.....