Your Drive Train ExplainedWritten by Kevin Schappell
The drive train serves two functions: it transmits power from engine to drive wheels, and it varies amount of torque. "Power" is rate or speed at which work is performed. "Torque" is turning or twisting force. Multiple ratio gearboxes are necessary because engine delivers its maximum power at certain speeds, or RPM (Rotations Per Minute). In order to use same engine RPM's at different road speeds, it is necessary to change "Gear Ratio" between engine and drive wheels. Just like a bicycle, car has to switch gears in order to move at a wide range of speeds. Unlike your bicycle, car's drivetrain also has to allow you to back up. (Well, you could push it backwards if you ate your Wheaties) There are actually two sets of gears in drive train; transmission and differential. The transmission allows gear ratio to be adjusted, and differential lets drive wheels turn at different speeds. Manual transmissions usually have four or five speeds, and often have "overdrive", which means that output shaft can turn faster than input shaft for fuel economy on highway. Some use an electric clutch and a switch that controls whether overdrive is engaged or not. An interesting development on a few cars is "clutchless" manual transmission, which uses a stick shift and an automatic electric clutch. Speed and position sensors, mini computers, and throttle controls keep engine from over-revving when driver shifts gears. As with many automotive "inventions", this is an old idea, which may now reach feasibility due to computer revolution. Automatic transmissions commonly use three forward gears to blend speed and torque. In case of a three-speed transmission, first gear delivers maximum torque and minimum speed for starting. Second gear offers medium torque and speed for acceleration and hill climbing. Third gear allows maximum speed with minimum torque for highway travel. A reverse gear permits backward movement. A transmission is a speed and power-changing device installed at some point between engine and driving wheels of a vehicle. It provides a means for changing ratio between engine RPM (Revolutions Per Minute) and driving wheel RPM to best meet each particular driving situation. Some types of drive train layouts use a "Transaxle", which is simply a combination of transmission and differential. These are usually found on front wheel drive cars, but are also used on mid- and rear-engine cars. Some exotic cars have their engine in front, and a transaxle in rear of car for better weight balance. Torque is derived from power. The amount of torque obtainable from a source of power is proportional to distance from center of rotation at which it is applied. It is logical, then, that if we have a shaft (in this case, crankshaft) rotating at any given speed, we can put gears of different sizes on shaft and obtain different results. If we put a large gear on shaft, we will get more speed and less power at rim than with a small gear. If we place another shaft parallel to our driving shaft and install gears on it in line with those on driving shaft, we can obtain almost any desired combination of speed or power within limits of engine's ability. That is exactly what an automobile transmission does by means of gears and other devices. There are two types of transmissions; manual and automatic. If you have a manual transmission, you have to shift gears yourself, usually with a stick located on your console and clutch pedal. If you have an automatic transmission, mechanism changes without any help from you. This is accomplished through a system that works by oil pressure. Each shift of gears is controlled by a shift valve; gears shift change depending on speed, road, and load conditions.
| | The Engine ExplainedWritten by Kevin Schappell
The engine is heart of your car, but instead of pumping blood, engine pumps air and fuel. The engines main function is to convert air and fuel into rotary motion so it can drive wheels of car. How does it do that ??.... Well let's start with a cutaway of engine and see all major parts then we will get into actual mechanics. Pistons: Most common engines have 4, 6, or 8 pistons, which move up and down in cylinders. On upper side of piston is what is called combustion chamber where fuel and air mix before ignited. On other side is crankcase, which is full of oil. Pistons have rings which serve to keep oil out of combustion chamber and fuel and air out of oil. Crankshaft: The crankshaft is connected to pistons via a connecting rod. As piston moves up and down in cylinder it rotates crankshaft and converts straight line motion into rotary motion. Valve train: The valve train consists of valves, rocker arms, pushrods, lifters, and camshaft. (shown in above picture in blue, yellow, and green) The valve train’s only job is that of a traffic cop. It lets air and fuel in and out of engine at proper time. The timing is controlled by camshaft, which is synchronized to crankshaft by a chain or belt. Now that we have a general overview of parts involved let's talk about what happens. Most automotive engine today are 4-stroke (or 4-cycle) engines, meaning they have four distinct events which make up cycle. ·Intake stroke: The camshaft opens intake valve and piston moves down cylinder. This creates vacuum and sucks in air and fuel into combustion chamber above piston. ·Compression stroke: As piston starts moving back up cylinder intake valve closes and seals off combustion chamber. The causes air and fuel to compress. ·Power stroke: As fuel is compressed and piston nears top of cylinder spark plug fires and ignites fuel and air. This explosion pushes piston back down cylinder and drives crankshaft. ·Exhaust stroke: After piston reaches bottom of cylinder, exhaust valve opens and gasses left over from fuel and air are sent out to exhaust system. Put these four events together in above order and you have a complete cycle. Are you asleep yet? That's enough theory, let's talk about real world and problems you might encounter with above mentioned parts. Pistons: Remember I talked about rings, which seal combustion chamber from crankcase. The rings over time tend to wear out. When they wear they allow fuel and air to enter into oil and dilute it. This dilution reduces oils ability to lubricate your engine and can cause premature wear. Also if rings wear down they can allow oil from crankcase to enter combustion chambers. This will result in oil being burned and exiting your tailpipe as grayish/white smoke. If your car spews grayish white smoke and it does not go stop in first few minutes after start-up you might have warn rings. If smoke goes away after start-up look to valve train section. Crankshaft: The crankshaft rides on bearings, which can wear down over time. The bearings support crankshaft and also rods, which connect pistons to crankshaft. A loud medium pitched knocking noise in engine points to warn bearings most of time. This is usually a costly repair and involves removing crankshaft and either machining surface where bearings ride, or replacing entire crankshaft. To prevent this type of problem, use a high quality oil, change your oil at suggested intervals (3 months or 3000 miles is a safe number) and always maintain your oil level between oil changes.
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