A History and Overview of the Trucking Industry Part 8

Figure 00-00, shows how a gasoline engine works.

• Intake stroke. To create a vacuum in the cylinder, the piston starts at the top of the stroke and moves downward. The intake valve will open to allow the air/fuel mixture to flow into the cylinder from the manifold.
• Compression stroke. During this phase, the intake and exhaust valves are closed in order to allow the fuel/air mixture to be compressed as the piston completes the upstroke. Compression ratio for gas engines generally range from 7:1 to 10:1, about half of that for a diesel engine. Combustion can occur at lower cylinder pressures because the fuel does not need to self-ignite, therefore very high compression ratios are not necessary with gas engines.
• Power stroke. Right before the piston begins its second downstroke, the spark plug at the top of the cylinder ignites the fuel/air mixture. Combustion then occurs and heat and pressure begin to build and push the piston downward, which results in crankshaft rotation.
• Exhaust stroke. Once the piston reaches the bottom of the power stroke, the exhaust valve opens and the piston begins to move upward again. This motion pushes the burnt gases out of the cylinder, through the exhaust manifold, exhaust pipe, and out into the atmosphere through the muffler. When the piston has reached the top of the cylinder, the cycle will begin again with the next intake stroke.

Many factors have to be thought of when deciding whether to use a diesel or gas engine in lighter-weight trucks. Diesel engines create a greater pulling torque which is necessary in hauling heavier weights, and operate at about half the engine revolution speed of gas engines. Even though this causes diesel engines to be more reliable and longer-lasting, it also requires the driver to shift gears more frequently because of a narrower torque band. Diesel engines also cost more than gas engines but they are more efficient, typically require less maintenance, consume less fuel and last longer. A diesel engine is noisier than a gas engine and emits dark exhaust smoke, whereas a gasoline engine runs more smoothly and puts out almost invisible exhaust gases.

Transmissions

The engine produces power which is transmitted to the wheels, which in turn forces to propel the vehicle forward.

A transmission enables the engine power to move the vehicle forward, backward, start from a stop, drive at highway speeds and climb steep hills. They are located between the engine and the drive wheels. The transmission allows the driver to pick a proper gear ratio to best match the engine torque and power to the demands of the road. The driver can use a low gear which will make the vehicle go at a low speed, but with great pulling ability, or the driver can use a high gear, which allows for fast speeds or driving on level ground.

To explain gear theory in simple terms, let’s say that there is one gear on the input shaft and the driver can select which of the several sized gears on the output shaft it will mesh with and turn. If a gear on the input shaft is meshed with a gear twice as big on the output shaft, the output shaft will turn ½ as fast and have twice the torque as the input shaft.

When moving a stopped truck, a large amount of torque and slow rotation is required at the drive wheels. To achieve this high torque, a low speed gear must be used. As the speed of the vehicle increases, the demand for torque decreases and the wheel speed must increase. The driver must progressively shift into higher gears (lower ratio gears) to increase the truck speed. Figure 00-00 depicts the complexity of a modern truck transmission.

A diesel engine’s advantage is that it generates the greatest amount of torque and horsepower through a narrow range of engine speeds. A gasoline engine is advantageous in that it operates at higher engine speeds and wider power bands than a diesel.

A diesel engine is designed to reach its max RPM at or just above the RPM which produces maximum horsepower. The transmission is supposed to be matched to the engine so that the engine RPM producing maximum engine torque and the max engine RPM. The engine is able to produce at least 85 to 90 percent of the maximum horsepower within this range. This allows the highest horsepower to fuel usage ratios.

Numerous gear ratios must be used in small steps in order to move the truck from rest to highway speeds. Gasoline engines have wider power and torque bands requiring fewer gears to move the vehicle from a stop to highway speeds.

Many factors must be considered when choosing which type of transmission to put into a diesel truck. The proper transmission will depend upon the torque and power features of the engine, the projected loads to be pulled, the projected vehicle cruising speed, the expected grades to be climbed up and down on, startability and how often it will stop and creep in heavy traffic.

A vehicle with heavy cargo will require a greater amount of torque at low speed to start to move, which requires low speed gear ratios. Another type of vehicle that would require lower gears for startability due to traction would be off-road equipment.

A vehicle whose primary use is for highway driving should have a transmission that is geared to maximum fuel economy at highway speeds. This would involve a gear ratio that results in the desired road speed when the speed is at a low fuel consumption point and is generating the right amount of torque.

In choosing the correct transmission, cost, availability of the components in the right size and the traditions of that line of truck or that industry must be considered. Two different types of transmissions available are manual, which require more effort on the driver’s part, and automatics, which can be more costly.

Manual Transmissions

An important issue about transmissions is that they are most efficient to keep an engine running within a narrow range of RPMs, regardless of what the road or load demands. In order for this to be achieved, a manual transmission main gear box typically has 3 to 7 forward gear ratios. More ratios are created by using multi-speed rear axles or by cascading two or more transmissions.

Smaller trucks, such as Class 6, usually have five forward gear ratios and variations from 4 to 7 are in existence. In order to improve startability, the truck may have a two-speed rear axle to decrease gear ratio step size. This results in a total of 10 speeds if used in a five speed transmission. There is a switch on the gear shift lever that a driver can use to control the rear axle ratio. Once the switch is turned on, the shift in the rear axle gear ratio works by an electric motor or an air cylinder moving the correct gears into place.

Larger trucks, such as Class 8, with diesel engines need additional gears with smaller ratio steps and usually have from 7 to 18 forward speeds. The transmissions in large trucks have a main gear box which generally has five to seven forward gear ratios and reverse.

When two or more transmissions are integrated together, they create additional gear ratios and are called a compound transmission.