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Commander
- Joined
- May 31, 2002
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Re: Fuel consumption on 21 foot Cabin boat with merc 4.3 or 5 Litre
It's the reason your car gets better MPG in overdrive than in high gear at 70 MPH (or any other constant speed) on the interstate. The engine has to deliver the same amount of power in overdrive as it does in high gear to cruise at 70 MPH. When it shifts into OD, the engine RPM must drop to stay at 70 MPH, although power must stay constant. Since Power=Torque x RPM, the engine must increase it's torque output to maintain the same power at the lower RPM. Only way to increase it's torque output is to open the throttle more. The bigger opening minimizes pumping losses so it takes less fuel to deliver the same amount of power.
In simple terms, with the butterflies cracked open under part throttle, the intake manifold is below atmospheric pressure. There is a partial vacuum in the intake manifold. That?s why you can use a vacuum gauge to diagnose problems with your motor. This vacuum is trying to pull the piston UP when it needs to go DOWN to pull in mixture and fill the cylinder. The extra fuel required to generate the power needed to overcome this part throttle vacuum reduces the efficiency of the engine. As the throttle is opened wider, there is less vacuum. The engine is more efficient because not as much extra fuel is needed to overcome vacuum losses. At full throttle, there is essentially no vacuum in the manifold.
There?s a basic explanation of p-V diagrams here:
http://www.grc.nasa.gov/WWW/K-12/airplane/pvtsplot.html
and a basic explanation of a part-throttle Otto cycle p-V diagram here:
http://www.mechadyne-int.com/vva-reference/part-load-pumping-losses-si-engine
We talked about a lot of this stuff in this thread a few years ago:
http://forums.iboats.com/showthread.php?t=225803&highlight=secrets
If they?re geared the same and both motors are designed to deliver the same max power, like in the Stingray test you referenced, I would expect the smaller motor to always win. Since they are going the same speed, they need to supply the same amount of power. The motors are turning the same RPM, so they have to produce the same amount of torque to produce the same power. But since the bigger motor has more AVAILABLE TORQUE (that?s why it can accelerate faster if you use full throttle), it must use a proportionally smaller throttle opening to limit the amount of torque delivered, thus making it more inefficient.
It?s only interesting when you compare different motors built for the same purpose with gearing/propping optimized for each. The Stingray test you referenced used a 30% bigger motor that made the same peak power as the smaller motor. It would have been designed for a heavy cruiser, not the performance boat they used it in. The article implied this in the last paragraph, where they say:
Basic thermodynamics there. It?s obvious if you know enough to understand a p-V diagram, not very intuitive otherwise. That?s why they?re trying to develop ?throttle-less? induction systems by controlling the valve lift for spark ignition engines. Unfortunately, very few boaters seem to understand it.QC said:
It's the reason your car gets better MPG in overdrive than in high gear at 70 MPH (or any other constant speed) on the interstate. The engine has to deliver the same amount of power in overdrive as it does in high gear to cruise at 70 MPH. When it shifts into OD, the engine RPM must drop to stay at 70 MPH, although power must stay constant. Since Power=Torque x RPM, the engine must increase it's torque output to maintain the same power at the lower RPM. Only way to increase it's torque output is to open the throttle more. The bigger opening minimizes pumping losses so it takes less fuel to deliver the same amount of power.
In simple terms, with the butterflies cracked open under part throttle, the intake manifold is below atmospheric pressure. There is a partial vacuum in the intake manifold. That?s why you can use a vacuum gauge to diagnose problems with your motor. This vacuum is trying to pull the piston UP when it needs to go DOWN to pull in mixture and fill the cylinder. The extra fuel required to generate the power needed to overcome this part throttle vacuum reduces the efficiency of the engine. As the throttle is opened wider, there is less vacuum. The engine is more efficient because not as much extra fuel is needed to overcome vacuum losses. At full throttle, there is essentially no vacuum in the manifold.
There?s a basic explanation of p-V diagrams here:
http://www.grc.nasa.gov/WWW/K-12/airplane/pvtsplot.html
and a basic explanation of a part-throttle Otto cycle p-V diagram here:
http://www.mechadyne-int.com/vva-reference/part-load-pumping-losses-si-engine
We talked about a lot of this stuff in this thread a few years ago:
http://forums.iboats.com/showthread.php?t=225803&highlight=secrets
If they?re geared and propped the same, that?s true. The Stingray test you referenced did exactly what you described above. It provided a good example of why you don't want to gear your big displacement engine like your small one. You want the big displacement engine to be turning slower than a small one at any given speed to take advantage of it's superior torque. Even though it?s turning at a lower RPM, it is still required to deliver the same amount of power to maintain the given speed, thus it must deliver more torque. If the big motor is geared so it?s turning slow enough, it can easily require a bigger throttle opening than the smaller motor to flow the same amount of air required to meet that power requirement. Thus it can have lower vacuum losses and be more efficient.QC said:
If they?re geared the same and both motors are designed to deliver the same max power, like in the Stingray test you referenced, I would expect the smaller motor to always win. Since they are going the same speed, they need to supply the same amount of power. The motors are turning the same RPM, so they have to produce the same amount of torque to produce the same power. But since the bigger motor has more AVAILABLE TORQUE (that?s why it can accelerate faster if you use full throttle), it must use a proportionally smaller throttle opening to limit the amount of torque delivered, thus making it more inefficient.
It?s only interesting when you compare different motors built for the same purpose with gearing/propping optimized for each. The Stingray test you referenced used a 30% bigger motor that made the same peak power as the smaller motor. It would have been designed for a heavy cruiser, not the performance boat they used it in. The article implied this in the last paragraph, where they say:
I would guess that the bigger motor in their test had a higher torque curve in the lower RPM range that was wasted in prop slip upon acceleration with the 21? prop they used. As discussed above, the higher torque curve required a smaller throttle opening (which decreased it?s efficiency) to deliver the same amount of power at the same RPM as the smaller motor. A better choice of motor for a comparison with the 300HP 350 would have been a properly geared/propped 385HP 454. It would have some of the torque moved towards the higher RPM range and required a bigger, more efficient throttle opening at lower RPM, but would also have had substantially more power at high RPM.
