There are two major components that must be carefully considered before making the choice between a gas engine shuttle bus and a diesel engine shuttle bus. Believe it or not, in some circumstances, it is better to run a gasoline powered shuttle bus than diesel. The first question a the purchaser must ask himself is, "What engine do I NEED in a bus?" Not "What engine do I WANT in a bus?"

This brings us to our first major component: You first must determine how often you are going to drive the shuttle. Some determinations are in how many miles/year and trips/month. A trip is considered each time it is started and driven, even if it was just moved to another parking spot.

In some cases you may be driving less than 10,000 miles per year but you are running the bus on a short route everyday. A diesel engine is then the preferred choice.

Also, important to note is the fuel economy. Either way you look at it, a diesel shuttle bus engine is more fuel efficient, getting about 92% efficiency from its fuel whereas gas engines are less fuel efficient, somewhere in the 80% rate. Therefore, diesels have better fuel mileage than gas.

Another note is the power. If you are going to run a bus with a full load of passengers, you are going to put a larger strain on your engine. This is what the diesel was built for: to maintain power with heavier loads during extended periods of time without heightened strain. A gasoline engine can lose power in a fully loaded 32 passenger bus.

The second major component is the cost. If you are going to be using the bus frequently, you must consider the more expensive option of a diesel. A diesel is more expensive to maintain due to the larger quantities of oil and other fluids and the size of the filters, which make them more expensive. The service intervals are longer than a gasoline engine, so your cost per mile will be slightly higher. However, the offset in fuel economy when under load as well as the length of service of the typical diesel should generate more return on the initial investment. As anyone who has ever pulled a heavy load with a gasoline engine knows, fuel economy drops dramatically. The diesel is a more efficient engine.

The reason why a diesel can produce more power with no added strain is because of the torque. Torque is a measure of the kinetic energy that builds up in a rotating engine. The higher the torque, the more power it takes to slow the engine down, in other words, it takes more power to make it work harder; therefore the engine will carry a heavier load with less strain.

Diesel engines develop more torque for several reasons. One is because of their greater mass: heavier parts generate more kinetic energy. The compression ratio, 3x as much as a gas engine, also generates more torque. Gasoline buss engines develop most of their horse power at the top end of their RPM curve; a diesel bus motor develops more power lower on the speed curve because of their greater torque, which can be thought of as the reserve power behind the rotating shaft.

Thus the diesel bus's great advantage is carrying more load with less strain on the engine due to the higher torque generated. When dealing with lighter passenger loads, that advantage disappears. Another advantage is that the diesel will develop that power with significantly less fuel. But that advantage is nullified by the much higher initial cost of the bus itself. The only real advantage is in the amount of fuel tank space savings since you can have smaller tanks with a diesel.

Another factor that is not thought of immediately is the resale value. Diesel buses will resell quicker and hold more value. They are in higher demand. Even though they cost more initially you will get it back at resale.