Engine and Drivetrain

Good performance and low lifecycle costs come from correctly matching components to application, followed by proper operation and maintenance


They note that warranty coverage of components is dictated by the vocation.

An understanding of the contribution of each individual component of the drivetrain will help as well, the ArvinMeritor officials say.

• Engine - The engine is used to generate enough power to provide the performance and fuel economy specified by the truck owner.

The officials recommend identifying the engine's "sweet spot" - the most efficient range of the engine chosen. The balance of power and performance does not necessarily have to mean a sacrifice, they say.

• Clutch - Its function is to connect and disconnect the power from the engine to the rest of the drivetrain. A properly adjusted clutch - making full contact - will eliminate loss from the engine and reduce heat.

• Transmission - The purpose of this component, simply put, is to transmit power to a driver mechanism. A manual transmission transmits power via an input shaft which splits the power (torque) inside the transmission box evenly between two countershafts and provides road speed via selected gears. The selected gears will be in reduction - direct or overdrive mode - to provide the road speed needed.

A direct-drive transmission will take less energy (parasitic loss) to move the power through the transmission. However, the direct drive may not have the top drive gear needed for a desired road speed.

An automatic transmission basically changes gear automatically, using a torque converter in lieu of a clutch. The torque converter multiplies the engine's torque.

An automated transmission combines features of manual and automatic transmissions. Automated shifting is controlled electronically (shift-by-wire) and performed by a hydraulic system or electric motor.

Engine torque and rpm, along with the load and desired road speed and driver issues, will determine the best transmission model.

• Driveshaft - This component, also known as the driveline, transmits torque and rotation. It is used to connect other components of a drivetrain that cannot be connected directly because of distance or the need to allow for relative movement between them. The driveline's mass should be effectively minimized so that energy from the engine is not lost, advise ArvinMeritor officials. The proper diameter of the shaft and length are factored in with engineering assistance from the OEM and driveline manufacturer.

• Rear drive axles - Their function is to take the engine power and turn it at 90-degree angles to drive the wheels and both wheel ends while allowing differentiation of wheel speed. Drive axles are ratio dependant on the vocation, application, cruise speed and transmission ratios selected for the job.

There will be some parasitic loss in the gears. However, a properly specified axle ratio that works in conjunction with the engine and transmission will give the desired road speed.

Axle ratio is the number of driveline revolutions necessary to turn an axle one time. By way of example, with an axle ratio of 3.50, the driveline will turn 3.50 times for each revolution of the axle. The higher the ratio, the more torque and the slower road speed for a given engine speed.

Common ratios for axles these days, according to ArvinMeritor officials, include 2.64 and 2.79 with a direct drive transmission and 3.21, 3.36 and 3.42 with an overdrive transmission.

If purchasing a stock truck, they stress the importance of being certain to match the truck's exact application to the right, or even special, componentry - a dump truck versus a pickup and delivery truck versus a linehaul truck.

Further, they recommend checking to be sure that the truck's gross vehicle weight rating and/or gross combination weight rating is sufficient for the operation's vocation. Also, be certain torque input does not exceed the axle's input rating.

Refer to the axle manufacturer's applications guidelines or contact the axle manufacturer, say ArvinMeritor officials. It is much better to do this before a truck is ordered, rather than after a failure and the warranty is denied.

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