Just some thoughts on RX-8 fuel economy
10-28-2004, 06:04 PM
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Originally Posted by PoorCollegeKid
Power per liter is a very misleading number. It really tells you nothing about the overall efficiency of the engine (as shown by the numbers you've quoted above) and, unless you are racing in a displacement limited class, it has no meaning when how well your car is going to perform assuming that it's gearing matches its powerband. You may as well pull out the power to door handles ratio, because that will tell you just as much about the performance of a vehicle as the power to displacement ratio will. For example, if you had to choose between a 200hp/L engine or a 70hp/L engine for your car, which one would you go with? I don't know about you, but I wouldn't be able to decide until I learned what the total output of the engines are and how much they weigh.
The most efficient engines in terms of fuel efficiency and thermodynamic efficiency tend to have rather low specific outputs. Take the Prius's engine, for example. The 1NZ-FXE displaces 1.5L and puts out 70 hp. If you look at its power:displacement ratio then you could conclude that this engine is rather inefficient, producing just 46.7hp/L. However, nothing could be farther from the truth, as we well know. Looking at the rotary's power:displacement ratio, you may be tempted to conclude that it is very efficient, when in actuality the LS2 that you compared it to is much more efficient from a power:weight standpoint and thermodynamically, two measures of efficiency that actually have some bearing on the performance of the engine even though it lags behind the rotary in the power:displacement ratio. Power:displacement is a nice marketing term and all, but to compare the efficiencies of engines by their power:displacement ratios is misguided at best.
The most efficient engines in terms of fuel efficiency and thermodynamic efficiency tend to have rather low specific outputs. Take the Prius's engine, for example. The 1NZ-FXE displaces 1.5L and puts out 70 hp. If you look at its power:displacement ratio then you could conclude that this engine is rather inefficient, producing just 46.7hp/L. However, nothing could be farther from the truth, as we well know. Looking at the rotary's power:displacement ratio, you may be tempted to conclude that it is very efficient, when in actuality the LS2 that you compared it to is much more efficient from a power:weight standpoint and thermodynamically, two measures of efficiency that actually have some bearing on the performance of the engine even though it lags behind the rotary in the power:displacement ratio. Power:displacement is a nice marketing term and all, but to compare the efficiencies of engines by their power:displacement ratios is misguided at best.
We are talking about the mechanical efficiency of the engines here, you are getting caught up on too many variables, forget racing dynamics, gear matching etc., just engines. The rotary generates more horsepower then the piston engine, the HP/L number quite clearly show's this and it's a viable measure. You say "I wouldn't be able to decide until I learned what the total output of the engines are and how much they weigh.", well the rotary also weighs less per HP generated.
Let's look at it this way, for the corvette to generate the same HP/L as the RX-8 it would have to be 14.9 L, what kind of gas mileage would it get then?
10-28-2004, 07:34 PM
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Originally Posted by Hellbreed
Why are you bringing a hybrid engine into this? of course it's fuel efficiency is going to better.
We are talking about the mechanical efficiency of the engines here, you are getting caught up on too many variables, forget racing dynamics, gear matching etc., just engines. The rotary generates more horsepower then the piston engine, the HP/L number quite clearly show's this and it's a viable measure. You say "I wouldn't be able to decide until I learned what the total output of the engines are and how much they weigh.", well the rotary also weighs less per HP generated.
Let's look at it this way, for the corvette to generate the same HP/L as the RX-8 it would have to be 14.9 L, what kind of gas mileage would it get then?
We are talking about the mechanical efficiency of the engines here, you are getting caught up on too many variables, forget racing dynamics, gear matching etc., just engines. The rotary generates more horsepower then the piston engine, the HP/L number quite clearly show's this and it's a viable measure. You say "I wouldn't be able to decide until I learned what the total output of the engines are and how much they weigh.", well the rotary also weighs less per HP generated.
Let's look at it this way, for the corvette to generate the same HP/L as the RX-8 it would have to be 14.9 L, what kind of gas mileage would it get then?
And, with regards to the 1NZ-FXE, I wasn't talking about fuel efficiency, I was talking about thermodynamic efficiency, which is one of the main ways that engineers and scientists measure the efficiency of an engine. The Prius's engine (the gasoline engine only, not counting the electric motor) has a thermodynamic efficiency of ~34%, which is much, much better than the average piston engine's 20%-25%. Click here for more info, if you like. Mechanically, piston engines are ~90%-95% efficient. The rotary is probably a little bit better than this due to fewer and lighter moving parts and therefore less frictional and inertial losses, but, unfortunately, this gain in mechanical efficiency is less than its loss in thermo efficiency, which is one of the reasons why rotaries are not known for their gas mileage. I can't find any hard numbers on the rotary's mechanical efficiency (or the Renesis's, specifically) so I really can't expound further on that topic.
The horsepower per liter ratio tells you nothing about the total amount of horsepower generated by an engine, so no, the rotary does not necessarily outpower a piston engine of lesser specific output like you say it will. Yes, it outpowers a 1.5L engine designed for efficiency, but does it outpower, say, Nissan's VQ35 that has a hp/L ratio of only 82? Power:displacement is mostly a marketing term and has very, very little to do with the engine's actual performance. If you know that an engine has a lot of power/liter and is NA, you could then assume that its torque output is a good deal less than an engine of equal power that has a lower power/liter rating or one that is turbo'd or s/c'd. Also, the higher the power/liter ratio is, the harder it is to squeeze more out from the engine through aftermarket parts. Remember, it's easier to get a 3 liter engine up to 120hp/L than it is to get a 2L, 240hp engine up to 3 liters. Besides very basic facts like these, hp/L in and of itself tells you nothing about an engine. The rotary has a great power:size ratio and is very unique, two of the many reasons that Mazda still uses these engines, but modern piston engines out do it in most every other way simply because they've been developed more and also have the benefit of a more efficient design when looking at them from a thermo standpoint. Don't get me wrong, I love the rotary, but I'm not going to ignore the benefits of another engine just because I happen to like the Renesis. Out of curiousity, how do you figure that the Corvette would need a 14.9L engine to equal the HP/L of the Renesis? I just can't figure out the math for that one
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