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Name: Muthu
Status: Student
Grade: 9-12
Location: Outside U.S.
Country: India
Date: Winter 2009-2010


Question:
How does an engineer calculate the ideal speed to get high mileage in a car or other vehicle?



Replies:
Hi Muthu,

The short answer to your question is "he doesn't!" Basically, there are far too many variables and unknowns to be able to calculate exactly what speed will give maximum mileage. Among the parameters that must be known are the following: Engine, transmission, and differential efficiencies at all speeds, detailed knowledge of aerodynamic drag at all speeds, rolling resistance of the tires and wheel bearings at all speeds, and so on.

As you can imagine, in order to attempt to accurately calculate the optimal speed for minimum fuel mileage, this quickly turns into an impossibly complex problem. As a result, engineers do not attempt to calculate this. Typically with most cars the speed that results in minimum fuel consumption is between 50 to 60 MPH (80 to 100 km/h). This is generally the speed where the engine runs most efficiently, and the speed just before air resistance (or "drag") starts to exponentially increase. To determine the answer to a greater accuracy, actual tests are made.

Regards,

Bob Wilson


It is complicated, and much of it is not actually calculated, but determined from experiments. The efficiency of a car moving at constant speed depends on many things, including engine design, drag (wind resistance), and friction from many sources. Let us just look at engine efficiency and drag, because their variation with car speed is significant.

A car's engine is optimized for a limited range of speeds, and geared so that the engine can stay mostly in that range. Above some engine speed, there is not enough time for the gas/air mixture to get into the cylinders and fill them optimally, or for the exhaust to get out optimally, so the efficiency drops. Below some engine speed, the pressure increase of ignited fuel in a cylinder is not well matched to the speed at which the moving piston increases the cylinder volume, so again the efficiency drops. (There are other factors, including precisely when the cylinders should best be ignited, that vary with engine speed. I mostly do not know what they are or how they vary with engine speed.)

In short, there is an optimal (most efficient) engine speed, and this means there is a different optimal car speed for each transmission gear.

Drag, on the other hand, increases steadily with car speed, but it is so hard to calculate accurately that wind-tunnel experiments are performed to measure the drag at different speeds. Experiments have shown that drag is negligible at low speed, and increases rapidly as speed increases.

Because drag increases with car speed, the best gas mileage should be achieved at some fairly low speed in first gear. (Nobody would actually drive at that speed, because they would hear honking and observe nasty gestures.) As the car speed increases, the efficiency will decrease because of engine inefficiency until the car is shifted to second gear. As the car's speed increases through the range of second gear, the efficiency will increase to a maximum and then begin decreasing. All the while the car speed is increasing, drag is increasing, so each gear's optimal speed will be lower than engine efficiency alone would suggest.

Tim Mooney


Muthu

There are two approaches, 1) by calculation; and 2) by measurement.

By calculation you can calculate the miles per gallon at different speeds

By measurement you actually measure how many miles per gallon a car gets at a given speed.

This measurement is actually determined for aircraft so if pilots have an engine failure in flight, they will know what speed they need to fly at for maximum range.

Sincere regards,
Mike Stewart



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