Diesel Engines: Lesson D 5: Timing Geartrains, Camshafts, Tappets, Rockers, and Cylinder Valves

pallet

a bearing surface designed to interconvert rotary and reciprocating motion; the pallet of a cylinder valve rides the valve stem or valve bridge.

valve float

a condition caused by running an engine at higher-than-specified rpms in which valve spring tension becomes insufficient, causing asynchronous (out of time) valve closing.

camshaft

a crankshaft-driven shaft, machined with eccentrics (cams) designed to actuate trains positioned to ride the cam profiles; the engine feedback assembly actuator responsible for timing/actuating cylinder valves and fuel injection apparatus. Driven at half engine speed on four-stroke cycle engines and at engine speed on two-stroke cycles.

spur gear

a gear with radial teeth.

helical gear

a gear with spiral cut teeth.

Tempilstick

a heat-sensing crayon used for precise determination of high temperatures.

valve bridge

a means of actuating a pair of cylinder valves with a single rocker; also known as a valve yoke.

key

a precision machined locating tab that slots into a mating keyway precision-machined into a shaft for the purpose of positioning a gear.

keyway

a precision machined slotted recess in a shaft for the purpose of inserting a key used for the purpose of precisely positioning a gear on the shaft.

train

a sequence of components with a common actuator.

scissor gear

a split section gear with one sector spring loaded to produce zero lash and quieter gear operation; used in more complex diesel engine timing geartrains.

valve polar diagram

a valve mapping exercise that makes use of circles or a spiral configuration to map the valve closing and opening events during the engine cycle.

tappets

a variety of devices that ride a cam profile and transmit the effects of the cam geometry to the train to be actuated; also known as followers.

clevis

a yoke machined with pin bosses with the objective of accommodating angular changes to a linkage.

valve train

all the components between the cam and the valve, which typically would include followers/tappets, push tubes/rods, rocker assemblies, and valve bridges/yokes.

balance shaft

an eccentric offset shaft designed to counter inherent imbalance dynamics in an engine; more common in 4-cylinder diesel engines.

valve

any switch device that can open, close, or moderate flow through a circuit. For instance, engine valves open and close so that its cylinders can breathe and seal.

lifters

components that ride a cam profile and convert rotary motion of the camshaft into linear motion or lift. Lifters used in truck diesel engines are generally solid or roller types.

valve margin

dimension between the valve seat and the flat face of the valve mushroom; critical valve machining specification.

concept gear

found in some diesel engine timing geartrains, a concept gear is a two-piece assembly that uses coaxial springs between the hub and outer toothed ring to maintain zero lash tooth contact with the gears it is in contact with.

periphery

in cam geometry, the entire outer boundary of the cam; cam profile.

inner base circle (IBC)

in cam geometry, the portion of the cam profile with the smallest radial dimension; also known as base circle/IBC. When the train riding the cam profile is on IBC, it is unloaded.

ramps

in cam geometry, the shaping of the cam profile between the IBC and the OBC. The ramp geometry defines the actuation/unload characteristics of the train that rides its profile.

rockers

shaft-mounted, pivoting levers that transmit the effects of cam profiles to valves and injection pumping apparatus.

keepers

split locks fitted to a peripheral groove at the top of a cylinder valve stem; hold the spring retainer in position.

overhead camshaft (OHC)

term used to describe a camshaft or camshafts mounted over the cylinder head(s). In most commercial diesels, OHCs are gear-driven.

companion cylinders

term used to describe pistons paired by their respective crank throws to rotate together through the engine cycle such as #1 and #6 in an inline, 6-cylinder engine.

overhead adjustment

term used to refer to setting cylinder head valves and timing injectors; also known as tune-up.

cam profile

the cam geometry; simply, the shape of the cam.

feedback assembly

the engine's mechanical self-management components, consisting of a geartrain, camshaft, valve trains, injector actuating trains, fuel injection pumping apparatus, and valves.

split locks

the keepers fitted to a peripheral groove at the top of a cylinder valve stem that hold the spring retainer in position.

outer base circle (OBC)

the portion of a cam profile with the largest radial diameter.

cam geometry

the shaping of a cam profile and the effect it produces on the train it actuates.

base circle (BC)

the smallest radial dimension of an eccentric. Used to describe cam geometry, the train that the cam is responsible for actuating would be unloaded on the cam base circle; also known as an inner base circle or IBC.

variable valve timing (VVT)

used by diesel engine OEMs to optimize power while minimizing emissions; ECM controlled and hydraulically actuated (engine lube) when used.

followers

used to describe a variety of devices that ride a cam profile and transmit the effects of the cam geometry to the train to be actuated; also known as a tappet.

interference angle

used to provide aggressive valve to valve seat bite; achieved by machining a valve seat at 1/2-degree less than cylinder head mating seat. Not used when valve rotators are used, therefore not common in diesel engines.