Gasoline Engine

1. Gasoline Engine Work • moving of an object against an opposing force by a push, pull or lift • measured in terms of distance and force, or foot-pounds (ft-Ib) in the USC • system or meter-kilograms (m-kg) in metric system Work = distance X force Example : 5 pound (2.27 kg) weight is lifted 5 feet (1.52 m)

2. Gasoline Engine Energy • the ability to do work. When work is done on an object, energy is stored in • the object • measured in joule Power • rate at which work is done – slowly or rapidly • usually measured as horsepower [hp] or as kilowatts [kW] in the metric system Torque • a turning or twisting force • engines produce power by turning a crankshaft in a circular motion • To convert terms of force applied in a straight line to a force applied in a • circular motion, the formula is Torque = force x radius

3. Gasoline Engine Inertia • cause an object to resist any change of speed or direction of travel • inertia must be overcome by applying power to make something move • inertia tries to keep the vehicle moving in a straight line. The tires must • overcome this tendency. Otherwise inertia may cause the vehicle to skid • off the road Friction • resistance to motion between two objects in contact with each other • the greater the load, the greater the friction and the greater force are required to • move the object. • there are 3 classes of friction: 1. Dry friction – resistance to motion between two dry objects 2. Greasy friction – friction between two objects thinly coated with oil or grease (occur in the engine when first starting.

4. Gasoline Engine Friction 3. Viscous friction – resistance to rotation between layers of liquid. In engine the layer of oil support the shaft and no metal to metal contact. Resistance only viscous friction.

5. Gasoline Engine Engine Torque • it results from combustion pressures pushing down the piston and it applies • torque through the connecting rod to the engine crankshaft. • higher volumetric efficiency produce a • higher combustion pressure and the • greater the torque is produce. • two valves engine start with higher • torque and drop as the engine speed • increase above 3000 rpm • four valves engine has a flatter curves • produce relatively higher torque

6. Gasoline Engine Engine Power • available from crankshaft to do work and measure in unit of horse power (hp). • It’s a measure of the rate at which a horse can work • a horse pull 200 lb for the distance • of 165 ft in 1 minute • amount of work performed are • 33,000 ft-lb Hp = L x W / 33,000 x t • engine power also measure in kilowatts (Kw) • One horse power is equal to 0.746 kW. • One kW is equal to 1.34 hp. • also can calculate if we know the engine torque and speed Hp = engine torque x rpm / 5252

7. Gasoline Engine Engine Power • there are several type of horsepower 1. Brake horse power (Bhp) – the power available from engine crankshaft to do work. 2. Gross horse power – measured by testing the basic engine 3. Net horse power – power delivered by a fully equipped engine 4. Road horse power – power delivered to the drive wheel. 5. Indicated horse power (Ihp) – power developed inside the combustion chamber 6. Friction horse power (Fhp) – power that required to overcome friction inside engine

8. Indicated thermal efficiency is the ratio of energy in the indicated power, ip, to the input fuel energy in appropriate units. • Indicated Thermal Efficiency (ηith) • ηith = ip/ E (energy in fuel [kW]) • ηith = bp/ E (energy in fuel [kW]) • ηm = bp/ip Brake Thermal Efficiency (ηbth) Brake thermal efficiency is the ratio of energy in brake power, bp, to the input fuel energy in appropriate units. Mechanical Efficiency (ηm) Mechanical efficiency is define as the ratio of brake power (delivered power) to the indicated power (power provided to the piston) • (ηm) = bp/ ip = bp/bp+fp • fp = ip – bp

9. Examples 1. The mechanical efficiency of a single-cylinder four-stroke engine is 80%. The friction power is estimated to be 25kW. Calculate the indicated power (ip) and brake power (bp) developed by the engine. Solution: bp/ip = 0.8 ip – bp = 25 (ip – 0.8) x ip = 25 ip = 25/0.2 = 125kW bp = ip – fp = 125 – 25 = 100kW

10. Examples 2. A 42.5kW engine has a mechanical efficiency of 85%. Find the indicated power and frictional power. If the frictional power is assumed to be constant with load, what will be the mechanical efficiency at 60% of the load? Solution: Indicated power, ip = bp/ ηm = 42.5/0.85 = 50 kW Frictional power, fp = ip – bp = 50 – 42.5 = 7.5 kW Brake power at 60% load = 42.5 x 0.6 = 25.5 kW Mechanical efficiency ηm = bp/ (bp + fp) = 25.5/ (25.5 + 7.5) =0.773 = 77.3%

11. efficiency means comparing the effort exerted with the results obtained. • Engine efficiency is relation between its actual power and its theoretical power • Engine Efficiency • 2 type of engine efficiency: • 1. mechanical efficiency • 2. thermal efficiency 1. Mechanical Efficiency the relationship between bhp and ihp Mechanical efficiency = bhp / ihp Example: At one speed, the bhp of an engine is 116. The ihp is 135 mechanical efficiency is 116/135 = 0.86, or 86 %. # This means that 86 % of the ihp is delivered by the engine. # The remaining 14 % is lost as fhp.

12. Gasoline Engine 2. Thermal Efficiency • the relation between the power produced and the energy in the fuel burned to • produce that power • Some of the heat produced by combustion • is carried away 1. by the engine lubricating / engine friction 2. by the cooling systems 3. lost in the hot exhaust gases as they leave the cylinder. 4. Lost in engine accessories • All these heat (thermal) losses reduce the • thermal efficiency of the engine. • Thermal efficiencies of spark-ignition engines may be below 20 percent. They are • seldom above 25 percent. Some diesel engines have thermal efficiencies of 35 % • or higher

13. Gasoline Engine Overall Efficiency • Energy is lost during transferring power to turning the drive wheels. Vehicle • are propelled by, about 20 % of the energy in the fuel. This energy is then • used up overcoming rolling resistance and air resistance. Volumetric Efficiency • Volumetric efficiency (VE) is the measure of how completely the cylinder fills • with air-fuel mixture during intake stroke. VE would be 100% if the cylinder • filled completely • However, several factors prevent this. 1. In a running engine, the air or air-fuel mixture must pass rapidly through narrow openings and bends in the intake manifold and cylinder head. 2. Engine heat warms the passing air and causes it to expand. • Intake valve opens for a short time for the cylinder to fill completely (only one • hundredth (0.01) second at high speed). 4. Exhaust gases that remain in the cylinder after the intake stroke begins allow less air-fuel mixture to enter

14. Gasoline Engine • Good volumetric efficiency for an engine running with the throttle wide open • at fairly high speed (3000 to 4000 rpm) is 80%. As speed increases, VE may • drop to 51%. This means the cylinders are only half-filled • There are several ways to improve volumetric efficiency. • Use larger intake valves. • Use more than one intake valve and exhaust valve per cylinder • Use forced induction system to pressurize the air or air-fuel mixture • Increasing valve lift - distance of valve moves down when it opens ( Vtec, VTC )

15. Gasoline Engine Reviewing Engine Performance • There are several can influence engine performance: 1. Piston displacements determine the volume of air or air-fuel mixture that will be enters the in induction stroke. Greater the displacement, more mixture can enter, and the more powerful the power strokes 2. Volumetric efficiency determines how much air-fuel mixture enter the cylinder at any speed. 3. Amount of mixture helps determine the pressure applied to the piston during the power stroke. More mixture, the higher the pressure, and the more powerful the power stroke 4. The pressure applied to the pistons determines engine torque 5. Torque and engine speed determine engine power