Knowledge Of Engine Mechanical Systems

Basic Engine Mechanical Information

Today’s cars and light-duty trucks use either gasoline, flex fuel (gasoline or E85) or diesel engines. By far the most common engine used in the United States is the gasoline piston engine. Flex fuel vehicles are generally regular production based gasoline engines with modifications to the fuel system and engine controls to support the use of E85 and gasoline. These modifications typically include larger fuel injectors, stainless steel fuel line components and computer reprogramming. Diesel engines have been very popular in lightand medium-duty pickups because of their high towing power and improved fuel economy over their gas counterparts. Many manufacturers are now offering clean diesel passenger cars that produce virtually no visible emissions and are very quiet. They offer dramatic improvements in gas mileage and the engines are very durable, lasting as much as twice as long as a similar gas engine. The engine provides the power to drive a vehicle’s wheels. Automobile and light truck engines, both diesel and gasoline, are classified as internal combustion engines. That is because the combustion that produces the power to drive the vehicle is produced inside the engine. Engines today are made of materials to make them as lightweight and efficient as possible. Examples of the type of materials they are made from are cast iron, aluminum, magnesium and fiber reinforced plastic. Engines are available with 3, 4, 5, 6, 8, 10 and 12 cylinders. These engines can be built with the cylinders arranged in-line, in a V or flat (cylinders opposed to each other). Engines can be located in different positions in the vehicle. In a RWD (Rear-Wheel Drive) vehicle, the engine is mounted longitudinally (crankshaft perpendicular to the drive axles) in the front of the vehicle. The transmission is attached to the rear of the engine and the differential is in the rear of the vehicle. In a FWD (Front-Wheel Drive) vehicle, the engine is mounted either longitudinally or transversely (crankshaft parallel to the drive axles) in the front of the vehicle. The engine is used with a transaxle (a transmission and differential combined) attached to it in the front. Some rearwheel drive vehicles have engines that are located midway between the passenger compartment and the rear axle. These engines can be mounted longitudinally or transversely and are usually connected to a transaxle.

Main Engine Components: Engine Block, Pistons, Connecting Rods & Crankshaft

The main components of an engine are the block, pistons, connecting rods, crankshaft, cylinder head and valvetrain. The block, the biggest part of an engine, is cast or machined with holes to provide passages for coolant and lubricants and moving parts. The cylinders are large, finely machined holes in the block that house the pistons. The pistons move up and down in the cylinders and resemble inverted cans. Sealing the piston to the cylinder wall is managed by piston rings called compression rings. They are specially shaped rings that fit into grooves in the piston. Pistons are also fitted with an oil control ring, which is located in a groove just below the compression rings. This ring acts as a scraper to remove excess oil from the cylinder wall that would otherwise be burned in the combustion chamber. Burned oil in the exhaust adds pollution to the air, and engines that burn oil lose power and efficiency. In order for the vehicle to move down the road, the up and down motion of the pistons must be converted to rotary motion. A connecting rod provides the link between the piston and the crankshaft. The upper end of the connecting rod is attached to the piston by a metal dowel called a wrist pin or piston pin. The big, lower end of the connecting rod attaches to the crankshaft. The crankshaft has connecting rod journals that are phased with crankshaft rotation. A 4-cylinder engine typically has rod journals that are 180 degrees apart so that two pistons are at the top of the cylinder and two are at the bottom at any given time. The reason this works is that engines in vehicles are 4 cycle, so that the two pistons that are up are actually in different parts of the 4 cycles. One piston has just completed the exhaust stroke while the other is starting the power stroke. V6 and V8 engines have 60° or 90° connecting rod pin separations, depending on their block types. The crankshaft is held in the engine block by main bearing caps. These caps hold the main bearings, which are made of soft metal and allow the crankshaft to turn while being lubricated by oil. The lower end of the connecting rod moves in a circular motion as the piston goes up and down, turning the crankshaft. The crankshaft is connected to the flywheel and the flywheel connects the engine to the drivetrain.

Automotive 4 Cycle Engines

As mentioned before automotive engines are 4 stroke whether they are gasoline or diesel. The first stroke is the intake stroke and begins with the piston all the way up at Top Dead Center (TDC). The intake valve will open before the piston reaches TDC so that the piston can draw air and the fuel provided by the fuel system into the cylinder. The intake valve closes just after the piston goes all the way down, reaching Bottom Dead Center (BDC). This starts the second stroke as the piston rises to compress the air and fuel mixture. A few degrees of crankshaft rotation prior to arriving at TDC again, the spark plug is fired to begin burning the air/fuel mixture (now very volatile because it is compressed 9 to 10 times its original size). The ensuing rapid expansion causes the power stroke, forcing the piston down. The last stroke is the exhaust stroke. As the piston comes back up, driven by inertia and its opposing cylinder, all the left over content of the cylinder is pushed out through the exhaust valve, which opens as the power stroke concludes. This is how the engine provides the energy to propel the vehicle down the road.

Diesel & Direct Injection

One slight variation exists with diesels and gas direct injection. Instead of the fuel being inside the intake manifold waiting for the intake valve to open, the fuel is actually sprayed under very high pressure directly at the piston, allowing more control and a longer power stroke. This type of fuel system will become the norm in years to come as it offers an instant increase in horsepower and fuel mileage and allows a reduction in vehicle emissions. Diesel engines are also known as compression ignition engines. They compress the air in the cylinder to 17-22 times its original size, which superheats it to the point where it becomes so hot that it ignites the fuel as it is injected into the cylinder. Diesels use some form of glow plug or intake air heater for cold starts. The cylinder head is mounted on top of the block and provides a sealed top to the cylinders. A head gasket goes between the head and block to prevent any internal or external leaks from the coolant and lubricant passages and the cylinders. The head and the top of each cylinder form the combustion chamber where the air/fuel mixture is compressed and burned. The cylinder head also contains the openings, called ports, for the air/fuel mixture to enter and the burned gases to leave the combustion chamber. Air is filtered prior to entering the intake system. The exhaust manifold provides the path to the exhaust system for the burned gases coming out of the cylinders. Gaskets are placed between the intake manifold, exhaust manifold and the engine to prevent leaks.

Learn More On Automotive Fuel Systems

Main Engine Components: Cylinder Head & Valvetrain

The valvetrain is made up of parts designed to open and close the intake and exhaust valves in the head. The valves, which look like mushrooms with flattened caps, are movable parts that open and close the ports in the head. The intake valve allows the air/fuel mixture into the combustion chamber for a gasoline-powered engine. Only air comes into the cylinder through the intake valve in a diesel engine. The exhaust valve allows the burned gases to leave the combustion chamber. The camshaft, a shaft with eccentric lobes on it, controls the opening and closing of the valves. There are two types of valvetrains; the OHV (Overhead Valve) type with the camshaft located in the block and the OHC (Overhead Camshaft) type with the camshaft located in the head. On the OHV engine, each cam lobe pushes a lifter (a small cylinder), which then pushes a pushrod, a thin metal rod that extends from the lifter into the head, where it fits into one end of a rocker arm. Each rocker arm, made of stamped or cast steel, is either mounted on a post or pivots on a shaft. Either way, each rocker arm behaves much like a seesaw, with the pushrod pushing up on one end while the other end pushes down on the valve stem, thereby opening the valve. Valve springs, placed around each valve, close the valves. OHC configurations include both Single Overhead Cam (SOHC) and Double (or Dual) Overhead Cam (DOHC) versions. These designations refer to the number of camshafts per cylinder head. DOHC engines have one camshaft for the intake valves and a second one for the exhaust valves. Thus, a DOHC in-line engine either four or six cylinders – will have two camshafts. A DOHC V6 or V8 will have four camshafts. OHC engines may open the valves directly, with only a lifter between the valve stem and the camshaft, or by rocker arms. Camshafts can be driven by timing chains or timing belts connected to the crankshaft. A few engines use timing gears. OHV engines typically use timing chains or gears, OHC engines may use timing belts, timing chains or a combination of both. Timing belts are toothed or cogged so that the camshaft remains in proper alignment with the crankshaft.

This post was written by: Martin Hand


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Martin Hand

About Martin Hand

ASE Certified L1 Advanced Mastertech. Martin Hand has over 15 years experience in Asian and European Import Auto Repair. Specializing in electrical diagnosis, engine performance, AT/MT transmission repair/rebuild. Martin is also pursuing a degree in Computers Science & Information Systems starting at Portland Community College while he plans to transfer to OIT. Certified in Java application level programming, experienced with other languages such as PHP, Ruby, JavaScript and Swift. Martin has future plans of automotive diagnostic software development.


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