Chapter 32 The Four-Stroke Cycle and Cylinder Arrangements

Introduction Engines produce torque, which turns the wheels Overview of how an engine works –Air-fuel mix is compressed in combustion chamber –Compressed mixture is ignited (for controlled burn) –Controlled burn elevates heat and pressure –Increased pressure drives pistons –Piston movement causes crankshaft to rotate

Engine Cycles Stroke: amount of piston travel from TDC to BDC –TDC: top dead center –BDC: bottom dead center Cycle: sequence that is repeated Four-stroke engine: four strokes make one cycle –Cycle repeats itself while automobile is running The intake stroke (first stroke of the cycle) –Piston moves from TDC to BDC –Vaporized air-fuel mixture is pushed into cylinder

Engine Cycles (continued) The compression stroke (second stroke of cycle) –The piston begins to return back to TDC –Air-fuel mixture is trapped, compressed, ignited The power stroke (third stroke of cycle) –Controlled burn causes molecules to expand –Expansion creates pressure that pushes piston down The exhaust stroke (fourth stroke of cycle) –As piston returns to BDC, exhaust valve is opened –Exhaust gas is pushed into the exhaust system

Cylinder Arrangements Number of cylinders determined by power needs Cylinders come in groups of 3, 4, 5, 6, 8, 10, or 12 –As cylinder numbers rise, engine performance rises –Adding more cylinders raises vehicle cost and weight Most common engine designs: –In-line: all cylinders are placed in a single row –Slant: type of in-line design with cylinders at a slant –V-type: two rows of cylinders set at 60 to 90 degrees –Opposed: two rows of cylinders facing each other

Valve Arrangements Overhead valve (OHV) –Intake and exhaust valves are in cylinder head –Camshaft and lifters are located in engine block Overhead cam (OHC) –Intake and exhaust valve, camshaft in cylinder head –Eliminates many moving parts of an OHV engine Dual overhead cam (DOHC) –Separate camshafts used for intake and exhaust –Can be either in-line or V-type

Diesel Engine Principles Similarities between diesel and gasoline engines –In-line and V-type configurations are available –Both engines can be two-stroke or four-stroke Features unique to diesel engines –Utilizes heat from compression to ignite fuel –Has very high compression ratio of 14:1 (600 psi) –Combustion chambers are adapted to diesel fuel

Vehicles with Alternate Power Sources Hybrid vehicles –Operate on two power sources –Typical sources: gasoline engine and electric motor –Example: Honda Insight Conventional electric vehicles –Batteries supply power to an electric motor Fuel cell vehicles –Cells produce electricity from hydrogen and oxygen –Electricity is delivered to motor (very low emissions)

Summary Combustion engine converts chemical energy in gasoline to mechanical energy that drives pistons The four strokes in a four-stroke engine: intake, compression, power, and exhaust Cylinder arrangements: V, slant, in-line, opposing Valve arrangements: OHV, OHC, and DOHC Other types of engines: diesel, hybrid, electric, and fuel cell