Under a Gibbous Moon

A new article from Wired showcases the 2011 Ford Mustang which comes standard with a 305 horsepower V6 and has an EPA rated highway mileage of 31.

One of the first comments struck me as interesting.

Posted by: fletc3her | 03/4/10 | 4:50 pm |

I think it’s great that the auto industry is starting to improve mileage even in sports cars and muscle cars. But, I do wonder what the actual day to day mileage of one these cars will be. If you push the car to get the advertised 0-60 speed then you are not going to get the advertised mileage. Less powerful cars force the driver to drive efficiently simply by virtue of not having enough horsepower to waste a lot of gas.

He says that less powerful cars force the driver to be more efficient because it simply does not have the power. Apparently he believes that an increase in horsepower necessitates a drop in fuel economy. It is true that they tend to be highly correlated but vehicle performance is more complicated than that.

For a brief primer, horsepower is a measurement  of work with work being force expended over a distance. The next big engine measurement is torque. Torque is a measure of instantaneous force and typically refers to a twisting action, in the case of an internal combustion engine, the crankshaft acting upon the input shaft of the transmission.

Torque is then multiplied through the gears in the transmission (or divided in the case of an overdriven gear). It is then multiplied one last time in the rear differential before being divided by the wheels (think of them as really big gears).

The power output of the engine varies with engine speed, torque reaching its peak before horsepower. When these peaks are reached varies from engine to engine but it is the gear multiplication that picks up the slack at low engine speeds.

The reason that this is important is that the engine is most efficient when it is producing the most power. This is why mileage in the “city” is always worse than “highway”, you are constantly running the vehicle through the lower and less efficient RPMs. An excellent example is watching the black smoke on a diesel from a stop. Diesels are very inefficient at low RPMs and the momentary black smoke is cause by a lot of unburnt fuel. Physics also comes into play, with regards friction and momentum (it takes a lot of energy to move a vehicle from a stop than it is to just keep going).

This brings me back to my earlier point about “forcing efficiency”. Smaller displacement engines (four cylinder and even smaller six cylinder) have very high power peaks four, five, and even six thousand RPMs. The little power that the engine does produce does not actually come into play for a long time. As stated earlier, this is overcome through the use of aggressive gearing. This has the dual purpose of multiplying torque and spinning up the engine faster.

The interesting thing about this, is that the smaller engine (in a similar weight and shaped vehicle) is working harder than a larger engine would. It uses less fuel simply by virtue of having a smaller displacement.

This comes back the Mustang mentioned in the beginning of this post. It (and the new Camero) are part of a new breed of small displacement, high power engines. While this is not a new trend (think of when the Mustang dropped the 5.0 for the 4.7). it has been accelerating due to rising fuel costs.

This extra power, gives you the ability to motivate the vehicle with less effort (fuel) which is why you are now seeing amazing performance in vehicles with mileage ratings once associated with compact cars.

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