Four-Stroke Engine Systems Flashcards

A technician is servicing a four-stroke engine that runs erratically under changing loads. Which component is most likely responsible for maintaining a consistent engine speed?

Answer: Governor system

The governor system is designed to automatically adjust the throttle to maintain a steady engine speed regardless of the load being placed on the engine. An erratic speed under changing loads points to a malfunction in this system.

During which phase of the four-stroke cycle are both the intake and exhaust valves momentarily open at the same time?

Answer: Valve overlap

Valve overlap is the period at the end of the exhaust stroke and the beginning of the intake stroke where both valves are open. This process uses the momentum of the exiting exhaust gases to help draw the new fuel/air mixture into the cylinder, improving engine efficiency at higher RPMs.

A customer's small engine will not start. A compression test reveals a reading of 40 PSI, which is significantly below the manufacturer's specification. Performing a 'wet' compression test, where a small amount of oil is added to the cylinder, results in the compression reading increasing to 95 PSI. What is the most likely cause of the low compression?

Answer: Worn piston rings

When oil is added to the cylinder, it temporarily creates a better seal between the piston rings and the cylinder wall. A significant increase in compression during a wet test indicates that the piston rings are worn and are the source of the compression loss.

Which of the following is a primary function of the flywheel in a four-stroke engine?

Answer: To store rotational energy

The flywheel's main purpose is to store kinetic energy from the power stroke. This stored energy maintains the crankshaft's rotation through the non-power strokes (exhaust, intake, compression), ensuring smooth engine operation.

In a splash lubrication system, how is oil primarily distributed to the various moving parts of the engine?

Answer: A dipper on the connecting rod splashes oil from the sump.

In a splash lubrication system, a dipper or slinger attached to the bottom of the connecting rod cap scoops up oil from the crankcase sump with each rotation of the crankshaft. This action splashes and throws oil onto the cylinder walls, bearings, and other moving parts.

When performing a compression test on a four-stroke engine, which of the following steps is essential for an accurate reading?

Answer: The throttle must be held wide open.

Holding the throttle plate wide open during a compression test is crucial. This allows the maximum amount of air to be drawn into the cylinder, ensuring the gauge measures the cylinder's true sealing capability without air restriction.