$ Stroke Engine Review Terms for Final

Camshaft

The brain of the engine. It works in conjunction with the crankshaft via a timing belt to make sure intake and outtake valves open and close at just the right time for optimal engine performance. The camshaft uses egg-shaped lobes that extend across it to control the timing of the opening and closing of the valves. Most camshafts extend through the top part of the engine block, directly above the crankshaft. On inline engines, a single camshaft controls both the intake and outtake valves. On V-shaped engines, two separate camshafts are used. One controls the valves on one side of the V and the other controls the valves on the opposite side. Some V-shaped engines (like the one in our illustration) will even have two camshafts per cylinder bank. One camshaft controls one side of valves, and the other camshaft controls the other side.

Cylinder Head

A piece of metal that sits over the engine's cylinders. There are small, rounded indentations cast into the cylinder head in order to create room at the top of the chamber for combustion. A head gasket seals the joint between the cylinder head and cylinder block. Intake and outtake valves, spark plugs, and fuel injectors (these parts are explained later) are also mounted to the cylinder head.

Power Stroke

An engine operation Stroke in which hot expanding gases force the piston head away from the cylinder head. Piston force and subsequent motion are transferred through the connecting rod to apply torque to the crankshaft. The torque applied initiates crankshaft rotation. The amount of torque produced is determined by the pressure on the piston, the size of the piston, and the throw of the engine. During the power Stroke, both valves are closed.

Four-Stroke Cycle Engine

An internal combustion engine that utilizes four distinct piston strokes (intake, compression, power, and exhaust) to complete one operating cycle. The piston make two complete passes in the cylinder to complete one operating cycle. An operating cycle requires two revolutions (720°) of the crankshaft. The four-stroke cycle engine is the most common type of small engine. A four-stroke cycle engine completes five Strokes in one operating cycle, including intake, compression, ignition, power, and exhaust Strokes.

Timing System

Holds the crankshaft and camshaft in the same relative position to each other at all times during the engine's operation. If the camshaft and crankshaft become out of sync for whatever reason (the timing chain skips a gear cog, for example), the engine won't work.

Why is an engine called a "V6" or "V8"?

It has to do with the shape and number of cylinders an engine has. In four-cylinder engines, the cylinders are typically mounted in a straight line above the crankshaft. This engine layout is called an inline engine. Another four-cylinder layout is called the "flat four." Here the cylinders are laid horizontally in two banks, with the crankshaft going down the middle. When an engine has more than four cylinders, they are divided into two cylinder banks — three cylinders (or more) per side. The division of cylinders into two banks makes the engine look like a "V." A V-shaped engine with six cylinders = V6 engine. A V-shaped engine with eight cylinders = V8 — four in each cylinder bank.

Engine Block (Cylinder Block)

The engine block is the foundation of an engine. Most engine blocks are cast from an aluminum alloy, but iron is still used by some manufacturers. The engine block is also referred to as the cylinder block because of the big hole or tubes called cylinders that are cast into the integrated structure. The cylinder is where the engine's pistons slide up and down. The more cylinders an engine has the more powerful it is. In addition to the cylinders, other ducts and passageways are built into the block that allow for oil and coolant to flow to different parts of the engine.

Valvetrain

The valvetrain is the mechanical system that's mounted to the cylinder head that controls the operation of the valves. The valve train consists of valves, rocker arms, pushrods, and lifters.

Valves

There are two types of valves: intake valves and outtake valves. Intake valves bring a mixture of air and fuel into the combustion chamber to create the combustion to power the engine. Outtake valves let the exhaust that's created after the combustion out of the combustion chamber. Cars typically have one intake valve and one outtake valve per cylinder. Most high-performing cars (Jaguars, Maseratis, etc.) have four valves per cylinder (two intake, two outtake). While not considered a "high performance" brand, Honda also uses four valves per cylinder on their vehicles. There are even engines with three valves per cylinder — two inlet valves, one outtake valve. Multi-valve systems allow the car to "breathe" better, which in turn improves engine performance.

Cam Lobes

These eccentric or egg-shapes cams on the camshaft are used to alternately open the valves, both intake and exhaust valves at the precise time in the cycle

Piston

They move up and down the cylinder. They look like upside down soup cans. When fuel ignites in the combustion chamber, the force pushes the piston downward, which in turn moves the crankshaft (see below). The piston attaches to the crankshaft via a connecting rod, aka the con rod. It connects to the connecting rod via a piston pin, and the connecting rod connects to the crankshaft via a connecting rod bearing. On the top of the piston, you'll find three or four grooves cast into the metal. Inside the grooves piston rings are put in. The piston rings are the part that actually touch the walls of the cylinder. They are made from iron and come in two varieties: compression rings and oil rings. The compression rings are the top rings and they press outward on the walls of the cylinder to provide a strong seal for the combustion chamber. The oil ring is the bottom ring on a piston and it prevents oil from the crankcase from seeping into the combustion chamber. It also wipes excess oil down the cylinder walls and back into the crankcase.

Ignition Event

This event occurs when the charge is ignited and rapidly oxidized through a chemical reaction to release heat energy. Combustion is the rapid, oxidizing chemical reaction in which a fuel chemically combines with oxygen in the atmosphere and releases energy in the form of heat. Proper combustion involves a short but finite time to spread a flame throughout the combustion chamber. The spark at the spark plug initiates combustion at approximately 20° of crankshaft rotation before TDC (BTDC). The atmospheric oxygen and fuel vapor are consumed by a progressing flame front. A flame front is the boundary wall that separates the charge from the combustion by-products. The flame front progresses across the combustion chamber until the entire charge has burned.

Exhaust Stroke

This occurs whenspent gases are expelled from the combustion chamber and released to the atmosphere. The exhaust stroke is the final stroke and occurs when the exhaust valve is open and the intake valve is closed. Piston movement evacuates exhaust gases to the atmosphere. As the piston reaches BDC during the power stroke combustion is complete and the cylinder is filled with exhaust gases. The exhaust valve opens, and inertia of the flywheel and other moving parts push the piston back to TDC, forcing the exhaust gases out through the open exhaust valve. At the end of the exhaust stroke, the piston is at TDC and one operating cycle has been completed.

Crankshaft

What converts the up and down motion of the pistons into a rotational motion that allows the car to move. The crankshaft typically fits lengthwise in the engine block near the bottom. It extends from one end of the engine block to the other. At the front of the end of the engine, the crankshaft connects to rubber belts which connect to the camshaft and delivers power to other parts of the car; at the back end of the engine, the camshaft connects to the drive train, which transfers power to the wheels. At each end of the crankshaft, you'll find oil seals, or "O-rings," which prevent oil from leaking out of the engine. The crankshaft resides in what's called the crankcase on an engine. The crankcase is located beneath the cylinder block. The crankcase protects the crankshaft and connecting rods from outside objects. The area at the bottom of a crankcase is called the oil pan and that's where your engine's oil is stored. Inside the oil pan, you'll find an oil pump that pumps oil through a filter, and then that oil is squirted on to the crankshaft, connecting rod bearings, and cylinder walls to provide lubrication to the movement of the piston stroke. The oil eventually drips back down into the oil pan, only to begin the process again Along the crankshaft you'll find balancing lobes that act as counterweights to balance the crankshaft and prevent engine damage from the wobbling that occurs when the crankshaft spins. Also along the crankshaft you'll find the main bearings. The main bearings provide a smooth surface between the crankshaft and engine block for the crankshaft to spin.

Intake Stroke

When the air-fuel mixture is introduced to fill the combustion chamber. The intake event occurs when the piston moves from TDC to BDC and the intake valve is open. The movement of the piston toward BDC creates a low pressure in the cylinder. Ambient atmospheric pressure forces the air-fuel mixture through the open intake valve into the cylinder to fill the low pressure area created by the piston movement. The cylinder continues to fill slightly past BDC as the air-fuel mixture continues to flow by its own inertia while the piston begins to change direction. The intake valve remains open a few degrees of crankshaft rotation after BDC. Depending on engine design. The intake valve then closes and the air-fuel mixture is sealed inside the cylinder.

Compression Stroke

When the trapped air-fuel mixture is compressed inside the cylinder. The combustion chamber is sealed to form the charge. The charge is the volume of compressed air-fuel mixture trapped inside the combustion chamber ready for ignition. Compressing the air-fuel mixture allows more energy to be released when the charge is ignited. Intake and exhaust valves must be closed to ensure that the cylinder is sealed to provide compression. Compression is the process of reducing or squeezing a charge from a large volume to a smaller volume in the combustion chamber. The flywheel helps to maintain the momentum necessary to compress the charge. When the piston of an engine compresses the charge, an increase in compressive force supplied by work being done by the piston causes heat to be generated. The compression and heating of the air-fuel vapor in the charge results in an increase in charge temperature and an increase in fuel vaporization. The increase in charge temperature occurs uniformly throughout the combustion chamber to produce faster combustion (fuel oxidation) after ignition. The increase in fuel vaporization occurs as small droplets of fuel become vaporized more completely from the heat generated. The increased droplet surface area exposed to the ignition flame allows more complete burning of the charge in the combustion chamber. Only gasoline vapor ignites. An increase in droplet surface area allows gasoline to release more vapor rather than remaining a liquid. The more the charge vapor molecules are compressed, the more energy obtained from the combustion process. The energy needed to compress the charge is substantially less than the gain in force produced during the combustion process. For example, in a typical small engine, energy required to compress the charge is only one-fourth the amount of energy produced during combustion. The compression ratio of an engine is a comparison of the volume of the combustion chamber with the piston at BDC to the volume of the combustion chamber with the piston at TDC. This area, combined with the design and style of combustion chamber, determines the compression ratio. Gasoline engines commonly have a compression ratio ranging from 6:1 - 10:1. The higher the compression ratio, the more fuel-efficient the engine. A higher compression ratio normally provides a substantial gain in combustion pressure or force on the piston. However, higher compression ratios increase operator effort required to start the engine. Some small engines feature a system to relieve pressure during the compression stroke to reduce operator effort required when starting the engine.

Combustion Chamber

Where fuel, air, pressure, and electricity come together to create the small explosion that moves the car's pistons up and down, thus creating the power to move the vehicle. The combustion chamber is made up of the cylinder, piston, and cylinder head. The cylinder acts as the wall of the combustion chamber, the top of the piston acts as the floor of the combustion chamber, and the cylinder head serves as the ceiling of the combustion chamber.