Engines

What is an internal combustion engine?

• Internal, in this case , means “inside” or “enclosed”.

• Combustion is the “act of burning”

• SO: An internal combustion engine burns fuel internally.

• ***An internal combustion engine turns heat energy into mechanical energy.

• For this to occur, there must be an exact mix of air and fuel.

• The air-fuel mixture expands rapidly while burning and pushes outward.

• This push can be used to move a part of the engine, and transmitted to drive the engine.

Elements of an internal combustion engine

• Air, Fuel, and Combustion• Reciprocating and Rotary Motion• Compression of Air-Fuel Mixture• Engine Cycles

Air, Fuel, Combustion

• Air is needed to combine with fuel and give it oxygen for fast burning.

• Air also:– Will compress – 1 cubic foot of air can be packed

into 1 cubic inch or less– Compressed air produces heat

Fuel

• Fuel must be readily be mixed with air to ignite easily

• 4 types of fuels:– Gasoline– LP gas– Natural gas– Diesel

• Fuels ignite more easily when vaporized. Why is this so?

Combustion

• Combustion is the actual igniting and burning of the air-fuel mixture. The oxygen combines with the vaporized fuel to provide the combustion.

• How fast the fuel burns determines the explosiveness needed to get full power from the engine

• We control the rate of burning by– How far we compress the air– How much fuel is used– How volatile the fuel is

Engine elements: Reciprocating and Rotary Motion

• Reciprocating motion = up and down or back and forth

• Rotary motion = a circular motion around a point

• Engines convert reciprocating motion to circular motion

• Four parts are needed: – Can you name them?

• Cylinder• Piston• Connecting rod• Crankshaft

• The piston fits very closely inside the cylinder, with the top of the cylinder being closed off by the cylinder head.

• The cylinder head has an extra space at the top (combustion chamber) where the combustion of the air-fuel mixture occurs.

• The connecting rod connects the piston to the crankshaft.

• When the piston is forced downward, it pushes the connecting rod down and forces the crankshaft to turn.

• This changes reciprocating motion the rotary motion.

• ** The crankshaft contains a section offset from the center line of the shaft. The offset is called the crank or “throw”.

• The stroke of the piston (how far it travels in the cylinder) is set by the throw of the crankshaft (how far it is offset).

Engine Elements: Compression of Air-fuel mixture

• An average gasoline engine works best when 14.7 parts of air are mixed with 1.0 parts of fuel, by weight.

• 14.7 pounds of air per 1 pound of fuel

• 1361 gallons of air per 0.16 gallons of fuel

Stoichiometric ratio

• 14.7 – to – 1 air to fuel ratio -- mixture

• Under perfect conditions, all of the fuel would be burned and all of the oxygen used up in the combustion process

• The stoichiometric ratio can vary and is dependent on octane and additives such at detergents and oxygenators.

AirAir

fuel

• Compression ratio – tells how much the air-fuel mixture is compressed by volume

• Example: if at the bottom position, the cylinder holds 8 pints, and at the top position, the cylinder holds 1 pint, the compression ration is 8 to 1.

Engine elements: Cycles

• Series of events:

• 1. Fill the cylinder with a combustible mixture• 2. Compress the mixture into a smaller space• 3. Ignite the mixture, causing it to expand and

produce power• 4. Remove the burned gases from the cylinder

• In other words:

• Intake (suck)• Compression (squeeze)• Power (bang)• Exhaust (blow)

• Thanks Decker!!

• Two stroke engines: Two strokes of the piston (one up and one down) during each cycle

• The whole cycle occurs during one revolution of the crankshaft.

• In four-stroke engines, the same 4 events occur (I – C – P – E), but with four strokes of the piston – two up and two down.

• The crankshaft will rotate twice to complete the cycle.

• A single cylinder only gives one power stroke every two revolutions of the crankshaft, thus it only produces power one-fourth of the time.

• Multiple cylinder engines allow the power stroke to follow much more closely, producing more continuous power.

Fuel systems

• Gasoline• LP – gas• Natural gas• Diesel

Gasoline fuel system

• The basic parts:

• Fuel tank – stores fuel• Fuel pump – moves fuel to carburetor• Carburetor – atomizes fuel and mixes w/ air

• Fuel supply systems– Gravity feed – has the fuel tank placed above the

level of the carburetor. Fuel flows by gravity to the carburetor.

– Force feed – fuel tank placed wherever necessary. A fuel pump moves fuel from the tank to the carburetor

LP Gas System

• LP is made up mainly of propane and butane that have been liquefied by compressing many gallons of vapor into one gallon of fluid.

• The fuel tank must be a closed unit to prevent the vapor from escaping.

• When starting the engine, vapor withdrawal is used because the heat exchanger cannot change liquid fuel to vapor (carb only operates on vapor)

• In the liquid withdrawal system, a heat exchanger converts liquid fuel to vapor by circulating hot water from the engine cooling system around the fuel line.

• As the fuel heats up and pressure is reduced, the fuel vaporizes.

Natural Gas Fuel System

• Natural gas is gaseous at any temperature above -259F. It must be stored in a closed container to prevent it from escaping.

• Natural gas must be stored as– Compressed Natural Gas (CNG)– Liquefied Natural Gas (LNG)

• LNG is stored in a high-strength insulated fuel tank to maintain the low temperatures.

Diesel fuel system

• In a diesel fuel system, fuel is sprayed directly into the engine combustion chamber where it mixes with hot compressed air and ignites.

• No electrical spark is used to ignite the mixture.

• Instead of a carburetor, a fuel injection pump and spray nozzle are used.

• Major components of a diesel engine– Fuel tank– Fuel transfer pump– Fuel filter– Water separator– Injection pump / high pressure pump– Injection nozzles

• Injection pump engines: the injection pump meters the fuel, puts the fuel under high pressure, then delivers it to each injection nozzle in the proper sequence.

• High pressure pump – the high-pressure pump delivers fuel under high pressure to common rail-type systems.

Intake System

• The intake system supplies the engine with clean air of the proper quantity, temperature, and fuel to provide the correct mixture for good combustion.

• The intake system has these 5 parts:– Air cleaner -- Intake manifold– Carburetor / air inlet -- Intake valves– Supercharger/turbocharger (optional)

• Air cleaners – remove dust and dirt from the air flowing to the intake manifold. A pre-cleaner prevents large particles from plugging the air cleaner

• Superchargers – pressurize the intake air. The supercharger can increase horsepower by packing more air (diesel) or air-fuel mix into the engine cylinders than the engine could take in by natural aspiration. Superchargers are gear or belt-driven and gets power from the crankshaft

• Turbochargers - perform a very similar function as the supercharger, but is driven by the engine’s exhaust.

• Intake manifold – route the air (diesel) or air-fuel mix to the engine cylinder

• Carburetor (gas) – mix incoming air with fuel in the proper proportion for combustion, and to control engine speed

• Intake valves – allow air / air-fuel into the combustion chamber. They are opened and closed by mechanical linkage from the camshaft

Exhaust system

• The exhaust system collects the exhaust gases after combustion and carries them away.

1. Removes heat2. Muffles engine sounds3. Carries away burned and unburned gases

• The exhaust system has 3 parts:1. Exhaust valves – opens to release burned

gases. – operated by the camshaft2. Exhaust manifold - collects the exhaust

gases and routes them away from the cylinder

3. Muffler – reduces the sound of the engine exhaust.

Lubrication system

• Responsible for– Reducing friction between moving parts– Absorbing and dissipating heat– Sealing the piston rings and cylinder walls– Cleaning and flushing moving parts– Deadening the noise of the engine

• Lubrication reduces friction between moving parts

• Lubrication also conducts heat away from moving parts

• The lubrication system may splash oil on the parts or feed oil under pressure via internal oil passages.

• The crankcase is where the oil is stored and cooled.

• The crankcase must be vented (back to intake) to prevent pressure from the blow-by gases from the piston

Cooling System

• Types: Air CoolingLiquid Cooling

• Air Cooling:– Uses air passing around the engine to dissipate

heat– Used mainly on small engines and aircraft

– Metal baffles, ducts, and blowers are used to aid in distributing air.

• Liquid cooling– Uses water or water-based solutions to dissipate

heat.– The water/coolant circulates around cylinders and

cylinder head– A large amount of engine heat is absorbed by

coolant, then flows to the radiator.– Air flowing through the radiator removes heat

from the coolant then dissipates the heat into the air.

• Water is an adequate coolant, but freezes in the winter

• It is mixed with ethylene glycol to prevent freezing.