Engine Size and Measurements Chapter 17

Objectives Describe various ways of measuring engine size Understand the effects of engine compression ratio Explain the principles of engine power and efficiency Relate torque to horsepower Understand the variations in operation of the different types of dynamometers

Introduction This chapter provides an understanding of: Various engine size and performance measurements Methods of understanding and measuring engine power output

Engine Size Measurements Engine’s size Determined by volume of air pistons displace in cylinders Cylinder bore size Diameter of the cylinder Usually three and a half to four inches Average engine has a three and a half to four inch stroke

Engine Size Measurements (cont’d.) Displacement Piston or cylinder displacement Volume displaced by the piston Bore2 x Stroke x 0.7854 Engine displacement Bore2 x Stroke x 0.7854 x number of cylinders

Compression Ratio Determines how much air and fuel are compressed Compression ratio is 8:1 Higher compression ratio increases power and fuel economy Increases exhaust emissions Each point of change is said to be worth about four to six percent change in horsepower Compression pressure Pressure made by piston moving up in cylinder Gasoline engines typically produce 125-175 psi

Compression Ratio (cont’d.) Effective compression ratio Determined by point at which intake valve closes

Physical Principles of Work Force: any action that tends to change the position of something Measured in pounds or Newtons Push, pull, or lift Work: when an object is moved against a resistant or opposing force Lifting or sliding English measurements: foot-pounds or watts Metric measurements: Newton-meters or joules Force x Distance = Work

Physical Principles of Work (cont’d.) One foot-pound: one pound is moved for a distance of one foot Energy: ability to produce motion against resistance Inertia: tendency of a body to keep its state of rest or motion Larger masses are affected more by inertia Momentum: product of the body’s mass and speed Power: how fast work is done

Torque Torque Tendency of a force to rotate a body on which it acts Amount of turning force exerted by crankshaft Engine torque varies with rpm High at lower speeds Heat Measured in Btu One Btu is the amount of heat required to heat one pound of water by one degree

Horsepower Measurement of an engine’s ability to perform work in a specified time One horsepower equals 33,000 foot-pounds of work per minute Amount of work required to life 550 pounds one foot in one second One horsepower equals 0.746 kilowatts Gross horsepower Power produced at crankshaft Several measurements

Horsepower (cont'd.) Accessories that rob power (absorb about 25%) Alternator Air conditioning Water pump Cooling fan Power steering Smog pump Net power is what remains Power is also lost through friction

Dynamometer Measures engine output Maximizing horsepower and torque Engine dynamometer Horsepower coming out of the engine Chassis dynamometer Horsepower available at vehicle’s drive wheels Maximizing horsepower and torque Valve overlap Period when intake and exhaust valves are open Scavenging Vacuum draws in fresh air and fuel through open intake valve

Dynamometer (cont’d.) Types of dynamometer power absorption units Automotive dynamometer power absorption units Electromagnetic and water brake Measuring torque and horsepower Horsepower: (Torque x rpm) / 5250 Torque readings: made at every 500 rpm Horsepower correction factors and comparisons Correction factors: compensate for high-altitude air or hot air Comparisons: done with same dynamometer

Dynamometer Safety Concerns Engine dynamometer Concerns: fire, part failure, and noise Chassis dynamometer Concerns: carbon monoxide, keeping the vehicle secured and connected to rollers, part failure, and noise Other dynamometer types Towing dynos Cycle dyne

Engine Efficiency Engine efficiency measurements: Amount of loss Mechanical efficiency Describes all ways friction is lost in the engine Engine output divided by engine input Volumetric efficiency Thermal efficiency Amount of loss Difference between efficiency measurement and 100%

Volumetric Efficiency Compares volume of airflow entering the engine with theoretical maximum Determines maximum torque output Changes with: Temperature Engine speed Load Throttle opening Increased speed and closing the throttle Lowered efficiency

Thermal Efficiency Ratio of how effectively an engine converts a fuel’s heat energy into usable work Gasoline’s thermal energy: 19,000 Btu per pound Brake thermal efficiency: more useful (Brake HP / fuel’s heat input) x 100 Spark ignition: only one-fourth of the energy from burning fuel is converted to work Diesel fuel: higher heat energy and engines have a higher compression ratio Better fuel economy

Mean Effective Pressure Pressure within the cylinder Increases during compression stroke Highest after ignition Peak cylinder pressure Should occur between ten and 20 degrees past TDC Combustion pressure moves the piston down Pressure drops as cycle continues