Gasoline Engines Operation

Energy and Power Energy is used to

produce power. Chemical energy is

converted to heat energy by burning fuel at a controlled rate. This is called

Combustion If engine combustion

occurs within the power chamber (combustion chamber) it is an internal combustion engine.

The chemical to heat energy conversion within the combustion chamber causes a pressure increase within the combustion chamber.

This pressure is applied to the head of the piston to produce mechanical force.

This mechanical force is converted into mechanical power.

Energy and Power

Four-Stroke Cycle Engines

All modern automotive engines utilize a four stroke design.

The four strokes are: Intake Compression Power Exhaust

Four-Stroke Cycle Engines Intake stroke - the

intake valve opens as the piston inside the cylinder travels downward. This draws the

air/fuel mixture into the cylinder

Compression stroke – the intake valve closes and the piston travels upward in the cylinder this compresses the

air/fuel mixture.

Four-Stroke Cycle Engines

Four-Stroke Cycle Engines Power stroke – as

the piston travels toward the top of the cylinder (top dead center) a spark plug ignites the compressed A/F mixture. This forces the

piston downward

Exhaust stroke – as the piston travels up tow This expels the spent

exhaust gasses

Four-Stroke Cycle Engines

The downward movement of the piston draws the air-fuel mixture into the cylinder through the intake valve on the intake stroke.

On the compression stroke, the mixture is compressed by the upward movement of the piston with both valves closed.

Ignition occurs at the beginning of the power stroke, and combustion drives the piston downward to produce power.

On the exhaust stroke, the upward-moving piston forces the burned gases out the open exhaust valve.

Crankshaft

Mounted at the “center-line” of the engine

The pistons are mounted to the crank via connecting rods

As the crank rotates the pistons move up and down in the engine bore

http://quadhub.com/wiki/images/a/ac/Cshaft.gif

720 Degree Cycle Each cycle of a four stroke engine requires two complete

revolutions of the crankshaft. The greater the number of cylinders, the more frequently

power strokes occur. This is why an eight cylinder engine runs smoother than a four

cylinder.

Engine Classification

Number of cylinders. Number of strokes. Cylinder arrangement

Inline engine V-type engine Horizontal (opposed)

boxer or pancake design

Automotive engine cylinderarrangements.

Engine Classification (cont.)

Longitudinal – the engine is mounted parallel with the length of the vehicle.

Transverse – the engine is mounted crosswise to the length of the vehicle.

Longitudinal front engine, rear-wheel drive.

Two types of front engine, front-wheel drive.

Valves and Camshafts

A traditional style of engine uses one intake and one exhaust valve per cylinder.

Newer designs use two intake and two exhaust valves per cylinder.

Valves and Camshafts

Valve opening and closing is performed by the camshaft.

Valves and Camshafts

If the camshaft is located in the engine block, valve operation will also utilize lifters, pushrods, and rocker arms. This arrangement is

called a pushrod engine.

Valves and Camshafts If the camshaft is

located over the valves, in the cylinder head, it is considered an overhead cam engine. Single overhead cam

(SOHC) engines utilize a common cam to operate the intake and exhaust valves per cylinder head.

Valves and Camshafts Double overhead

cam (DOHC) engines utilize a separate camshaft for the intake and exhaust valves per cylinder head.

Note: a v-type DOHC engine will have four camshafts per engine!

Cam Operation

Bore & Stroke The diameter of the cylinder

is the bore. The distance that the piston

travels within the bore between TDC and bottom dead center (BTC) is the stroke. The longer the stroke, the

greater the amount of air-fuel mixture that can be drawn into the cylinder. The greater the A/F

mixture, the greater the force when the mixture is ignited.

The bore and stroke of pistons are used to calculate an engine’s displacement.

TECHTIP

Engine Displacement

Displacement is generally referred to in terms of cubic inches (cu. in.) cubic centimeters (cc) or liters (l).

Displacement = the volume of the cylinder x the number of cylinders.

Displacement = B x B x S x 0.7854 x # cyls.

Engine Displacement

An over-square displacement engine has a larger bore than stroke.

Transversely, an under-square engine has a larger stroke than bore.

Over-square displacement

Engine Displacement

Engine Displacement

If an engine is said to be ‘bored” or “bored-out” the stock engine bore has been increased.

If an engine is said to be “stroked” the stock engine stroke has been increased. This is achieved at the crankshaft

Either event effectively increases the stock displacement of the engine.

Engine Displacement Formula

Bore X Bore X Stroke X 0.7854 X number of cylinders

For example, take a 6-cylinder engine where

Bore = 4.000 in., Stroke = 3.000 in.

Applying the formula,

4.000 in. X 4.000 in. X 3.000 in. X 0.7854 X 6 = 226 cu. in.

Engine Displacement

If an engine is bored larger diameter pistons must be installed.

If an engine is stroked the height of the piston must be compensated for and the engine may need to be “clearanced”. This is generally done by decreasing the length of

the connecting rod … And/or raising the wrist pin bore in the piston.

Compression ratio is the ratio of the total cylinder volume (when the piston is at the bottom of its stroke) to the clearance volume (when the piston is at the top of its stroke).

Compression Ratio

= PV+DV+GV+CV

DV+GV+CV PV = piston volume DV = deck clearance volume GV = head gasket volume CV = combustion chamber volume (pg.298)

Combustion chamber volume is the volume above the piston with the piston at top dead center.

The Four Stroke Cycle Video

Torque

Torque = twisting force Torque = the amount of force multiplied by

the length of the lever through which it acts. Torque is measured in foot pound (ft. lbs.) or

Newton-meters (N-m).

Torque is a twisting force equal to the distance from the pivot times the force applied expressed in units called pound-feet (lb-ft) or Newton-meters (N-m).

Work & Power

Work = the applied force x distance of movement

Power = the rate of doing work If a 550 lb. object is moved one foot in 10 seconds

or 10 minutes the same amount of work is performed.

More power is exerted to move the object in ten seconds.

Horsepower

Horsepower = the power required to move 550lbs. one foot in one second.

Or 33,000lbs. one foot in one minute. Horsepower = torque x RPM / 5,252 SAE gross H.P vs. SAE net H.P. pg.58.

One horsepower is equal to 33,000 foot-pounds (200 lbs. X 165 ft.) of work per minute.

TECHTIP

H.P. vs. Torque

Engine Power Video

Engine Aspiration

If the air-fuel mixture is introduced into the cylinder solely through vacuum it is considered to be a naturally aspirated engine.

Turbocharged – an exhaust driven turbine forces the A/F mixture into the engine.

Supercharged – an engine driven air-pump. Roots-type centrifugal

Engine Aspiration Turbocharged – an exhaust driven turbine forces the A/F

mixture into the engine. Exhaust driven

Engine Aspiration

Supercharged – an engine driven air-pump. Crankshaft driven

Turbochargers and Superchargers

A turbocharged (exhaust-driven) system is designed to provide a pressure greater than atmospheric pressure in the intake manifold. This increased pressure forces additional amounts of air into the combustion chamber over what would normally be forced in by atmospheric pressure. This increased charge increases engine power.

The amount of “boost” (or pressure in the intake manifold) is measured in pounds per square inch (psi), inches of mercury (in. Hg), in BAR’s, or in atmospheres. The higher the level of boost (pressure), the greater the horsepower potential.

A supercharged system is an engine-driven system designed to provide pressure greater than atmospheric pressure in the intake manifold. A supercharger is used on some engines from the factory including the General Motors 3800 cc, V-6, which uses an Eaton-built unit.