Transcript Slide 1
Photo courtesy DaimlerChrysler 2003 Jeep® Grand Cherokee Engine (Internal Combustion Engine) Parts of an Engine Spark plug The spark plug supplies the spark that ignites the air/fuel mixture so that combustion can occur. The spark must happen at just the right moment for things to work properly. Valves The intake and exhaust valves open at the proper time to let in air and fuel and to let out exhaust. Note that both valves are closed during compression and combustion so that the combustion chamber is sealed. Piston A piston is a cylindrical piece of metal that moves up and down inside the cylinder. Piston rings Piston rings provide a sliding seal between the outer edge of the piston and the inner edge of the cylinder. The rings serve two purposes: They prevent the fuel/air mixture and exhaust in the combustion chamber from leaking into the sump during compression and combustion. They keep oil in the sump from leaking into the combustion area, where it would be burned and lost. Most cars that "burn oil" and have to have a quart added every 1,000 miles are burning it because the engine is old and the rings no longer seal things properly. Connecting rod The connecting rod connects the piston to the crankshaft. It can rotate at both ends so that its angle can change as the piston moves and the crankshaft rotates. Crank shaft The crank shaft turns the piston's up and down motion into circular motion just like a crank on a jack-in-the-box does. Sump The sump surrounds the crankshaft. It contains some amount of oil, which collects in the bottom of the sump (the oil pan). The Engine Within the incredibly small space of time the cylinders of the engine must, depending on which of the working strokes is being performed in them, be filled with fresh mixture, must ignite and burn this mixture, must allow the burnt mixture to expand or must expel the burnt gases to the exhaust. In view of this extremely rapid succession of events, the fresh mixture for example rushes into the cylinder at speeds which can exceed 300 km/h, and the burnt gas is expelled at about the same speed. In a typical car engine running at 4800 rpm. 2400 of these working cycles must take place in each cylinder every minute to precisely the same degree of accuracy. During each complete working cycle, the temperature in the cylinder varies between 100 and up to 2500 °C, and the gas pressure varies between a slight vacuum and 40 bar or even higher. It is therefore obvious that engine components are exposed to very severe, fluctuating thermal and mechanical loads. The exhaust valve heads, for example, operate at red heat (at more than 600 °C) since they are exposed directly to the hot burnt gases, but must nevertheless withstand severe mechanical loads as the valves are opened and closed. The combustion pressure of about 40 bar acts on each piston, which depending on its diameter may have to withstand a force of 14000 to 30000 N. In other words, each piston incurs a load between one and a half and twice the weight of the complete car up to 2400 times every minute. This clearly indicates the high demands placed on materials, manufacturing accuracy, assembly and maintenance where the reliability and efficiency of highspeed motor-vehicle engines concerned. The Basics The purpose of a gasoline car engine is to convert gasoline into motion so that your car can move. Currently the easiest way to create motion from gasoline is to burn the gasoline inside an engine. Therefore, a car engine is an internal combustion engine -- combustion takes place internally. Almost all cars currently use what is called a four-stroke combustion cycle to convert gasoline into motion. The fourstroke approach is also known as the Otto cycle, in honor of Nikolaus Otto, who invented it in 1867. The four strokes are illustrated in Figure 1. They are: Intake stroke Compression stroke Combustion stroke Exhaust stroke Figure 1 Here's what happens as the engine goes through its cycle: The piston starts at the top, the intake valve opens, and the piston moves down to let the engine take in a cylinder-full of air and gasoline. This is the intake stroke. Only the tiniest drop of gasoline needs to be mixed into the air for this to work. (Part 1 of the figure) Then the piston moves back up to compress this fuel/air mixture. Compression makes the explosion more powerful. (Part 2 of the figure) When the piston reaches the top of its stroke, the spark plug emits a spark to ignite the gasoline. The gasoline charge in the cylinder explodes, driving the piston down. (Part 3 of the figure) Once the piston hits the bottom of its stroke, the exhaust valve opens and the exhaust leaves the cylinder to go out the tail pipe. (Part 4 of the figure). The Diesel Cycle Rudolf Diesel developed the idea for the diesel engine and obtained the German patent for it in 1892. His goal was to create an engine with high efficiency. Gasoline engines had been invented in 1876. Photo courtesy DaimlerChrysler Atego six-cylinder diesel engine The main differences between the gasoline engine and the diesel engine are: A gasoline engine intakes a mixture of gas and air, compresses it and ignites the mixture with a spark. A diesel engine takes in just air, compresses it and then injects fuel into the compressed air. The heat of the compressed air lights the fuel spontaneously. A gasoline engine compresses at a ratio of 7:1 to 12:1, while a diesel engine compresses at a ratio of 14:1 to as high as 25:1. The higher compression ratio of the diesel engine leads to better efficiency. Gasoline engines generally use either carburation, in which the air and fuel is mixed long before the air enters the cylinder, or port fuel injection, in which the fuel is injected just prior to the intake stroke (outside the cylinder). Diesel engines use direct fuel injection -- the diesel fuel is injected directly into the cylinder. Note that the diesel engine has no spark plug, that it intakes air and compresses it, and that it then injects the fuel directly into the combustion chamber (direct injection). It is the heat of the compressed air that lights the fuel in a diesel engine. piston + connecting rod A piston is a sliding plug that fits closely inside the bore of a cylinder. Its purpose is either to change the volume enclosed by the cylinder, or to exert a force on a fluid inside the cylinder. The piston can be divided up into the following distinct areas: crown, ring zone, body or skirt and gudgeon pin bushings. The piston crown can be fiat or slightly convex or concave. On high-performance engines, a recess acting as part or all of the combustion chamber is often formed in the piston crown. The connecting rod has two principal tasks to fulfil: it connects the piston to the crankshaft and, since its lower or "big" end is attached to an offset crankpin on the crankshaft, it converts linear movement of the piston into rotary movement of the crankshaft. By doing so, it transforms the linear force of the piston into a rotary force or torque. The connecting rod is exposed to very severe loads. Combustion gas pressure acting downwards on the piston exerts very heavy forces along the connecting rod. Since the piston's speed is continually varying, the high acceleration and deceleration forces which result must be withstood in the form of tensile and compressive loads on the connecting rod. Furthermore, the oscillating movement of the connecting rod round the gudgeon pin axis introduces powerful bending forces into the rod. and since the rod is fairly long is also subject to buckling stresses. The task of the crankshaft in a motor vehicle engine is to convert the linear force exerted by the pistons and transmitted by the connecting rods into rotary motion, so that the force becomes a torque. Most of this torque is passed on to the clutch or transmission of the vehicle, but a small proportion is needed to drive the valve gear, the oil pump, the distributor, the fuel supply system, the cooling system and the generator. In addition, the flywheel absorbs a certain amount of torque. The crankshaft must be capable of withstanding severe loads. The pistons and connecting rods accelerate and decelerate on every single stroke, that is to say each time the crankshaft rotates once. This results in very high dynamic forces. In addition, the crankshaft incurs severe centrifugal forces. The combination of all these forces means that the crankshaft is subjected to torsional, bending and torsional oscillation stresses, as well as incurring wear at the bearing points. How Automobile Ignition Systems Work Spark Plug The spark plug is quite simple in theory: It forces electricity to arc across a gap, just like a bolt of lightning. The electricity must be at a very high voltage in order to travel across the gap and create a good spark. Voltage at the spark plug can be anywhere from 40,000 to 100,000 volts. The spark plug must have an insulated passageway for this high voltage to travel down to the electrode, where it can jump the gap and, from there, be conducted into the engine block and grounded. The plug also has to withstand the extreme heat and pressure inside the cylinder, and must be designed so that deposits from fuel additives do not build up on the plug. Spark plugs use a ceramic insert to isolate the high voltage at the electrode, ensuring that the spark happens at the tip of the electrode and not anywhere else on the plug; this insert does double-duty by helping to burn off deposits. Ceramic is a fairly poor heat conductor, so the material gets quite hot during operation. This heat helps to burn off deposits from the electrode. The Coil The coil is the device that generates the high voltages required to create a spark. It is a simple device -- essentially a high-voltage transformer made up of two coils of wire. One coil of wire is called the primary coil. Wrapped around it is the secondary coil. The secondary coil normally has hundreds of times more turns of wire than the primary coil. The Distributor The distributor handles several jobs. Its first job is to distribute the high voltage from the coil to the correct cylinder. This is done by the cap and rotor. The coil is connected to the rotor, which spins inside the cap. The rotor spins past a series of contacts, one contact per cylinder. As the tip of the rotor passes each contact, a high-voltage pulse comes from the coil. The pulse arcs across the small gap between the rotor and the contact (they don't actually touch) and then continues down the spark-plug wire to the spark plug on the appropriate cylinder. When you do a tune-up, one of the things you replace on your engine is the cap and rotor -- these eventually wear out because of the arcing. Also, the spark-plug wires eventually wear out and lose some of their electrical insulation. This can be the cause of some very mysterious engine problems. Older distributors with breaker points have another section in the bottom half of the distributor -- this section does the job of breaking the current to the coil. The ground side of the coil is connected to the breaker points. A cam in the center of the distributor pushes a lever connected to one of the points. Whenever the cam pushes the lever, it opens the points. This causes the coil to suddenly lose its ground, generating a high-voltage pulse.