Reciprocating Engines practice questions

Answer: A) Operate at relatively high cruise power so cylinder pressures help seat the piston rings

Unless the manufacturer says otherwise, a newly overhauled engine is broken in at fairly high cruise power so cylinder pressures seat the rings. Prolonged low-power running is avoided, and straight mineral oil is used during break-in.

• ACorrect. Higher cylinder pressures push the new rings against the cylinder walls so they seat properly. The manufacturer's break-in instructions take precedence.
• BIncorrect. Long periods at low power do not load the rings enough. This can glaze the cylinder walls and stop the rings from seating.
• CIncorrect. Aggressive leaning during break-in risks overheating the cylinders and damaging new parts. Break-in is normally done with a richer mixture.
• DIncorrect. Straight mineral oil is normally used during break-in, because some ashless dispersant oils may slow the seating of the rings against the cylinder walls.

Answer: C) Peak pressure occurs just after TDC

• AIncorrect. That isn't the reason.
• BIncorrect. Valve timing is separate.
• CCorrect. Combustion takes time to build pressure.
• DIncorrect. Fuel enters during intake.

Answer: B) A compression tester reading of the load at a specified compressed height

Valve spring strength is checked with a valve spring compression tester. It measures the load the spring produces at a specified compressed height, and that load is compared with the overhaul limits.

• AIncorrect. A dial indicator at the rocker arm measures valve lift or runout. It does not measure the force a spring produces.
• BCorrect. A valve spring compression tester compresses the spring to the height given in the overhaul data and reads the load. A spring that falls below the minimum load is rejected.
• CIncorrect. A differential compression test checks how well the cylinder holds air past the rings and valves. It cannot isolate or measure valve spring force.
• DIncorrect. A feeler gauge measures valve clearance (lash). It does not tell whether the spring produces its required load.

Answer: C) A fouled spark plug in a cylinder fired by the left magneto

An excessive RPM drop on only one magneto points to a fault in that magneto's circuit, such as a fouled plug or bad lead. Conditions like mixture or induction icing affect both magnetos the same way.

• AIncorrect. Mixture affects combustion in every cylinder no matter which magneto is firing. Both magneto checks would show a similar change, not just one.
• BIncorrect. Induction icing lowers power for the whole engine. You would see it as an RPM loss on both magnetos, not an excessive drop on only one.
• CCorrect. With only the left magneto running, that cylinder fires on its fouled plug alone and misses, so RPM drops too far. On the right magneto the other plug in that cylinder works, so that drop is normal.
• DIncorrect. An open P-lead keeps the magneto from being grounded, so it stays 'hot'. That shows up as no drop, or a failure to shut off, not an excessive drop.

Answer: A) Automatically maintain zero clearance in the valve operating mechanism

Hydraulic (zero-lash) valve lifters use engine oil pressure to keep zero clearance in the valve operating mechanism automatically. Valve springs and the cam profile still control valve closing, lift, and timing.

• ACorrect. Engine oil pressure fills the lifter's plunger chamber and takes up any clearance automatically. The valve train runs at zero lash as parts expand and contract with temperature.
• BIncorrect. Valve lift and duration are set by the cam lobe profile. A hydraulic lifter only removes clearance; it does not change lift or timing with rpm.
• CIncorrect. A fixed cold clearance belongs to solid lifters. Hydraulic lifters remove the need for running clearance, although dry lifter clearance is checked at assembly.
• DIncorrect. Valve springs still close the valves and keep them seated. The hydraulic lifter only fills the gap between the cam and the push rod.

Answer: C) Usually below 100%

• AIncorrect. That would need boost.
• BIncorrect. Losses reduce it.
• CCorrect. Intake restrictions and heat reduce the charge.
• DIncorrect. It measures the air charge.

Answer: A) Advanced timing

• ACorrect. A spark before TDC at cranking speed can drive the piston backward.
• BIncorrect. This does not cause kickback.
• CIncorrect. This is unrelated.
• DIncorrect. This causes hard starts, not kickback.

Answer: B) A dead or weak cylinder

• AIncorrect. This is unrelated.
• BCorrect. A cylinder running cooler than the others is not firing properly.
• CIncorrect. This is unrelated.
• DIncorrect. This is unrelated.

Answer: A) To avoid dry-start wear

• ACorrect. Pre-oiling fills passages so bearings are lubricated from the first turn.
• BIncorrect. This is not the purpose.
• CIncorrect. Priming uses fuel.
• DIncorrect. This is unrelated.

Answer: B) Area × stroke × cylinders

• AIncorrect. RPM is not part of displacement.
• BCorrect. Displacement is piston area times stroke times number of cylinders.
• CIncorrect. This is not displacement.
• DIncorrect. Valves are not part of it.