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How Car Engine Work Part 3

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How Car Engine Work Part 3

Post by LhYnxz on Sun Mar 02, 2008 5:45 pm

Engine Cooling, Air-intake and Starting Systems

The cooling system in most cars consists of the radiator and water pump. Water circulates through passages around the cylinders and then travels through the radiator to cool it off. In a few cars (most notably Volkswagen Beetles), as well as most motorcycles and lawn mowers, the engine is air-cooled instead (You can tell an air-cooled engine by the fins adorning the outside of each cylinder to help dissipate heat.). Air-cooling makes the engine lighter but hotter, generally decreasing engine life and overall performance. See How Car Cooling Systems Work for details.

Diagram of a cooling system showing how all the plumbing is connected

So now you know how and why your engine stays cool. But why is air circulation so important? Most cars are normally aspirated, which means that air flows through an air filter and directly into the cylinders. High-performance engines are either turbocharged or supercharged, which means that air coming into the engine is first pressurized (so that more air/fuel mixture can be squeezed into each cylinder) to increase performance. The amount of pressurization is called boost. A turbocharger uses a small turbine attached to the exhaust pipe to spin a compressing turbine in the incoming air stream. A supercharger is attached directly to the engine to spin the compressor.

Photo courtesy Garrett
See How Turbochargers Work for details.
Increasing your engine's performance is great, but what exactly happens when you turn the key to start it? The starting system consists of an electric starter motor and a starter solenoid. When you turn the ignition key, the starter motor spins the engine a few revolutions so that the combustion process can start. It takes a powerful motor to spin a cold engine. The starter motor must overcome:

  • All of the internal friction caused by the piston rings
  • The compression pressure of any cylinder(s) that happens to be in the compression stroke
  • The energy needed to open and close valves with the camshaft
  • All of the "other" things directly attached to the engine, like the water pump, oil pump, alternator, etc.

Because so much energy is needed and because a car uses a 12-volt electrical system, hundreds of amps of electricity must flow into the starter motor. The starter solenoid is essentially a large electronic switch that can handle that much current. When you turn the ignition key, it activates the solenoid to power the motor.
Next, we'll look at the engine subsystems that maintain what goes in (oil and fuel) and what comes out (exhaust and emissions).

Engine Lubrication, Fuel, Exhaust and Electrical Systems

When it comes to day-to-day car maintenance, your first concern is probably the amount of gas in your car. How does the gas that you put in power the cylinders? The engine's fuel system pumps gas from the gas tank and mixes it with air so that the proper air/fuel mixture can flow into the cylinders. Fuel is delivered in three common ways: carburetion, port fuel injection and direct fuel injection.

  • In carburetion, a device called a carburetor mixes gas into air as the air flows into the engine.
  • In a fuel-injected engine, the right amount of fuel is injected individually into each cylinder either right above the intake valve (port fuel injection) or directly into the cylinder (direct fuel injection).

See How Fuel Injection Systems Work for more details.
Oil also plays an important part. The lubrication system makes sure that every moving part in the engine gets oil so that it can move easily. The two main parts needing oil are the pistons (so they can slide easily in their cylinders) and any bearings that allow things like the crankshaft and camshafts to rotate freely. In most cars, oil is sucked out of the oil pan by the oil pump, run through the oil filter to remove any grit, and then squirted under high pressure onto bearings and the cylinder walls. The oil then trickles down into the sump, where it is collected again and the cycle repeats.
Now that you know about some of the stuff that you put in your car, let's look at some of the stuff that comes out of it. The exhaust system includes the exhaust pipe and the muffler. Without a muffler, what you would hear is the sound of thousands of small explosions coming out your tailpipe. A muffler dampens the sound. The exhaust system also includes a catalytic converter. See How Catalytic Converters Work for details.
The emission control system in modern cars consists of a catalytic converter, a collection of sensors and actuators, and a computer to monitor and adjust everything. For example, the catalytic converter uses a catalyst and oxygen to burn off any unused fuel and certain other chemicals in the exhaust. An oxygen sensor in the exhaust stream makes sure there is enough oxygen available for the catalyst to work and adjusts things if necessary.
Besides gas, what else powers your car? The electrical system consists of a battery and an alternator. The alternator is connected to the engine by a belt and generates electricity to recharge the battery. The battery makes 12-volt power available to everything in the car needing electricity (the ignition system, radio, headlights, windshield wipers, power windows and seats, computers, etc.) through the vehicle's wiring.
Now that you know all about the main engine subsystems, let's look at ways that you can boost engine performance.

Producing More Engine Power

HorsepowerFor a complete explanation of what horsepower is and what horsepower means, check out How Horsepower Works.
Using all of this information, you can begin to see that there are lots of different ways to make an engine perform better. Car manufacturers are constantly playing with all of the following variables to make an engine more powerful and/or more fuel efficient.
Increase displacement - More displacement means more power because you can burn more gas during each revolution of the engine. You can increase displacement by making the cylinders bigger or by adding more cylinders. Twelve cylinders seems to be the practical limit.
Increase the compression ratio - Higher compression ratios produce more power, up to a point. The more you compress the air/fuel mixture, however, the more likely it is to spontaneously burst into flame (before the spark plug ignites it). Higher-octane gasolines prevent this sort of early combustion. That is why high-performance cars generally need high-octane gasoline -- their engines are using higher compression ratios to get more power.
Stuff more into each cylinder - If you can cram more air (and therefore fuel) into a cylinder of a given size, you can get more power from the cylinder (in the same way that you would by increasing the size of the cylinder). Turbochargers and superchargers pressurize the incoming air to effectively cram more air into a cylinder. See How Turbochargers Work for details.
Cool the incoming air - Compressing air raises its temperature. However, you would like to have the coolest air possible in the cylinder because the hotter the air is, the less it will expand when combustion takes place. Therefore, many turbocharged and supercharged cars have an intercooler. An intercooler is a special radiator through which the compressed air passes to cool it off before it enters the cylinder. See How Car Cooling Systems Work for details.
Let air come in more easily - As a piston moves down in the intake stroke, air resistance can rob power from the engine. Air resistance can be lessened dramatically by putting two intake valves in each cylinder. Some newer cars are also using polished intake manifolds to eliminate air resistance there. Bigger air filters can also improve air flow.
Let exhaust exit more easily - If air resistance makes it hard for exhaust to exit a cylinder, it robs the engine of power. Air resistance can be lessened by adding a second exhaust valve to each cylinder (a car with two intake and two exhaust valves has four valves per cylinder, which improves performance -- when you hear a car ad tell you the car has four cylinders and 16 valves, what the ad is saying is that the engine has four valves per cylinder). If the exhaust pipe is too small or the muffler has a lot of air resistance, this can cause back-pressure, which has the same effect. High-performance exhaust systems use headers, big tail pipes and free-flowing mufflers to eliminate back-pressure in the exhaust system. When you hear that a car has "dual exhaust," the goal is to improve the flow of exhaust by having two exhaust pipes instead of one.
Make everything lighter - Lightweight parts help the engine perform better. Each time a piston changes direction, it uses up energy to stop the travel in one direction and start it in another. The lighter the piston, the less energy it takes.
Inject the fuel - Fuel injection allows very precise metering of fuel to each cylinder. This improves performance and fuel economy. See How Fuel Injection Systems Work for details.
In the next sections, we'll answer some common engine-related questions submitted by readers.

Engine Questions and Answers

Here is a set of engine-related questions from readers and their answers:

  • What is the difference between a gasoline engine and a diesel engine? In a diesel engine, there is no spark plug. Instead, diesel fuel is injected into the cylinder, and the heat and pressure of the compression stroke cause the fuel to ignite. Diesel fuel has a higher energy density than gasoline, so a diesel engine gets better mileage. See How Diesel Engines Work for more information.
  • What is the difference between a two-stroke and a four-stroke engine? Most chain saws and boat motors use two-stroke engines. A two-stroke engine has no moving valves, and the spark plug fires each time the piston hits the top of its cycle. A hole in the lower part of the cylinder wall lets in gas and air. As the piston moves up it is compressed, the spark plug ignites combustion, and exhaust exits through another hole in the cylinder. You have to mix oil into the gas in a two-stroke engine because the holes in the cylinder wall prevent the use of rings to seal the combustion chamber. Generally, a two-stroke engine produces a lot of power for its size because there are twice as many combustion cycles occurring per rotation. However, a two-stroke engine uses more gasoline and burns lots of oil, so it is far more polluting. See How Two-stroke Engines Work for more information.
  • You mentioned steam engines in this article -- are there any advantages to steam engines and other external combustion engines? The main advantage of a steam engine is that you can use anything that burns as the fuel. For example, a steam engine can use coal, newspaper or wood for the fuel, while an internal combustion engine needs pure, high-quality liquid or gaseous fuel. See How Steam Engines Work for more information.
  • Are there any other cycles besides the Otto cycle used in car engines? The two-stroke engine cycle is different, as is the diesel cycle described above. The engine in the Mazda Millenia uses a modification of the Otto cycle called the Miller cycle. Gas turbine engines use the Brayton cycle. Wankel rotary engines use the Otto cycle, but they do it in a very different way than four-stroke piston engines.
  • Why have eight cylinders in an engine? Why not have one big cylinder of the same displacement of the eight cylinders instead? There are a couple of reasons why a big 4.0-liter engine has eight half-liter cylinders rather than one big 4-liter cylinder. The main reason is smoothness. A V-8 engine is much smoother because it has eight evenly spaced explosions instead of one big explosion. Another reason is starting torque. When you start a V-8 engine, you are only driving two cylinders (1 liter) through their compression strokes, but with one big cylinder you would have to compress 4 liters instead.

How Are 4-Cylinder and V6 Engines Different?

The number of cylinders that an engine contains is an important factor in the overall performance of the engine. Each cylinder contains a piston that pumps inside of it and those pistons connect to and turn the crankshaft. The more pistons there are pumping, the more combustive events are taking place during any given moment. That means that more power can be generated in less time.

4-Cylinder engines commonly come in “straight” or “inline” configurations while 6-cylinder engines are usually configured in the more compact “V” shape, and thus are referred to as V6 engines. V6 engines have been the engine of choice for American automakers because they’re powerful and quiet but still light and compact enough to fit into most car designs.

Historically, American auto consumers turned their noses up at 4-cylinder engines, believing them to be slow, weak, unbalanced and short on acceleration. However, when Japanese auto makers, such as Honda and Toyota, began installing highly-efficient 4-cylinder engines in their cars in the 1980s and 90s, Americans found a new appreciation for the compact engine. Even though Japanese models, such as the Toyota Camry, began quickly outselling comparable American models, U.S. automakers, believing that American drivers were more concerned with power and performance, continued to produce cars with V6 engines. Today, with rising gas prices and greater public environmental awareness, Detroit seems to be reevaluating the 4-cylinder engine for its fuel efficiency and lower emissions.

As for the future of the V6, in recent years the disparity between 4-cylinder and V6 engines has lessened considerably. In order to keep up with the demand for high gas-mileage and lower emission levels, automakers have worked diligently to improve the overall performance of V6 engines. Many current V6 models come close to matching the gas-mileage and emissions standards of the smaller, 4-cylinder engines. So, with the performance and efficiency gaps between the two engines lessening, the decision to buy a 4-cylinder or V6 may just come down to cost. In models that are available with either type of engine, the 4-cylinder version can run up to $1000 cheaper than the V6. So, regardless of what kind of performance you’re looking to get out of your car, the 4-cylinder will always be the budget buy.

One final note: It’s not a good idea to try to install a V6 engine into a car model that comes with a standard 4-cylinder. Retrofitting a 4-cylinder car to handle a V6 engine could cost more than simply buying a new car.

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