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Old 10-03-2007, 02:05 PM   #31
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22s playa1
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I think if i could get that type of FE i would have no problem driving a dildo shaped car.
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Old 10-03-2007, 02:45 PM   #32
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Pardon the Interruption

Sorry to interrupt the wheel discussion fun...

I'm pretty certain that larger wheels and tires may become more costly to purchase/maintain and even power...

Back to the unsprung weight argument I made earlier (in some thread ) -- lesser-expensive wheels generally become heavier as they get larger, which translates into more energy given during acceleration unless momentum is conserved. I guess it depends on the type of driving...

But also, to fit larger tires on most cars, you have to go with a more aggressive aspect-ratio -- then you're back to the original total diameter -- plus more $ for those types of tires.

I'd rather swap a transmission or gear, albeit more difficult...

RH77
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Old 10-03-2007, 08:41 PM   #33
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Sounds like a gud exkuse to get mas t00ls!
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I think if i could get that type of FE i would have no problem driving a dildo shaped car.
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Old 10-04-2007, 07:33 AM   #34
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Engine efficiency, for everything from semi engines to small gasoline engines is a function of load and engine speed. High load and low engine speed are typically where efficiency is the best. Although, low load hurts efficiency more than high engine speed does IMLE. Efficiency is given in terms of grams of fuel used per kWh generated, so lower is better. Here are some examples. . .
Thank you for the explanation. This is very instructive. One question: I understand that the vertical axis represents load on the engine, but is load the horsepower being demanded of the engine or just the torque? If I'm reading the maps correctly, I'm assuming that the vertical axis is only the torque component since horsepower is torque x rpm and rpm is represented separately on the horizontal axis. Am I correct?

So if a car needs say 20 hp to cruise down the highway at a given speed, then lowering the rpm (with taller gearing) increases the torque required in order to maintain the 20 hp output.

Now since the fuel consumption circles represent fuel quantity (grams, gallons, etc.) per hour of power output (hp, KW, etc.), then running the engine at a lower rpm that demands more torque (i.e., load), up to a certain point, to make that power gives the best efficiency.

So let's say again that you want to run at a speed that requires 20 hp. Since hp = torque x rpm x 1/5250, then at 2,000 rpm the engine would have to make 52.5 ft-lbs. If you run at 3,000 rpm, then the engine only has to make 35 ft-lbs. Now let's look at your fuel consumption map for the prius engine, which has the best high rev fuel efficiency of any of the engines you've shown. Making 20 hp at 2,000 rpm and 52.5 ft-lbs (71 Nm), fuel consumption is less than 240 g/kWh. At 3,000 rpm and 35 hp (47.5 Nm), fuel consumption goes up to 270 g/kWh. So even though the prius engine has very "flat" fuel consumption lines across the rpm range, as rpm increases, the engine load required to maintain a given horsepower drops, so engine efficiency drops. In other words, if you plot 20 hp on the map at different rpms, you get a line sloping down from left to right. So plot a given hp at various rpms on any of the fuel maps that you've provided and you find that the best efficiency comes in around 2,000 rpm or less.

Am I reading things correctly here?
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Old 10-04-2007, 11:17 AM   #35
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The X axis is engine speed and Y-axis air/torque, with peak hp corresponding to peak torque at whatever rpm. Everything you wrote seems to be accurate. Regarding the gearing line for different ratios at a given power/speed output and efficiency, realistically, it can be anywhere from 1000-4000rpm depending on displacement and engine. An older small displacement SOHC engine designed to rev high and make power may have it relatively high, while a newer large displacement DOHC may have it around idle.
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Old 10-04-2007, 02:37 PM   #36
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. . . and Y-axis air/torque . . .
How do you convert from bmep [bar] to ft-lbs torque?
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Old 10-04-2007, 04:19 PM   #37
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I just find the engine's peak torque and scale it from there. I suppose we could also use the eff, energy content of fuel, stoich ratio, and percent of oxygen at sea level for gassers, but I'm not sure about how we could do the same for diesels w/o knowing the amount of fuel injected...
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