Tagged: How Car Engines Work
Sep 18, 2019 at 4:48 pm #39684The 2018 Mercedes-AMG G65 final edition engine delivers 621 hp and 738 lb-ft. of torque. MERCEDES AMG
Have you ever opened the hood of your car and wondered what was going on in there? A car engine can look like a big confusing jumble of metal, tubes and wires to the uninitiated.
You might want to know what’s going on simply out of curiosity. Or perhaps you are buying a new car, and you hear things like “2.5-liter incline four” and “turbocharged” and “start/stop technology.” What does all of that mean?
In this article, we’ll discuss the basic idea behind an engine and then go into detail about how all the pieces fit together, what can go wrong and how to increase performance.
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.
Two things to note:
- There are different kinds of internal combustion engines. Diesel engines are one type and gas turbine engines are another. Each has its own advantages and disadvantages.
- There is also the external combustion engine. The steam engine in old-fashioned trains and steam boats is the best example of an external combustion engine. The fuel (coal, wood, oil) in a steam engine burns outside the engine to create steam, and the steam creates motion inside the engine. Internal combustion is a lot more efficient than external combustion, plus an internal combustion engine is a lot smaller.
Let’s look at the internal combustion process in more detail in the next section.
Sep 18, 2019 at 4:52 pm #39685
The principle behind any reciprocating internal combustion engine: If you put a tiny amount of high-energy-density fuel (like gasoline) in a small, enclosed space and ignite it, an incredible amount of energy is released in the form of expanding gas.
You can use that energy for interesting purposes. For example, if you can create a cycle that allows you to set off explosions like this hundreds of times per minute, and if you can harness that energy in a useful way, what you have is the core of a car engine.
Almost every car with a gasoline engine uses a four-stroke combustion cycle to convert gasoline into motion. The four-stroke 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
The piston is connected to the crankshaft by a connecting rod. As the crankshaft revolves, it has the effect of “resetting the cannon.” 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 tailpipe. (Part 4 of the figure)
Now the engine is ready for the next cycle, so it intakes another charge of air and gas.
In an engine, the linear motion of the pistons is converted into rotational motion by the crankshaft. The rotational motion is nice because we plan to turn (rotate) the car’s wheels with it anyway.
Now let’s look at all the parts that work together to make this happen, starting with the cylinders
Sep 18, 2019 at 4:54 pm #39686
Basic Engine PartsFigure 2. Inline: The cylinders are arranged in a line in a single bank.
The core of the engine is the cylinder, with the piston moving up and down inside the cylinder. Single cylinder engines are typical of most lawn mowers, but usually cars have more than one cylinder (four, six and eight cylinders are common). In a multi-cylinder engine, the cylinders usually are arranged in one of three ways: inline, V or flat (also known as horizontally opposed or boxer), as shown in the figures to the left.
So that inline four we mentioned at the beginning is an engine with four cylinders arranged in a line. Different configurations have different advantages and disadvantages in terms of smoothness, manufacturing cost and shape characteristics. These advantages and disadvantages make them more suitable for certain vehicles.Figure 3. V: The cylinders are arranged in two banks set at an angle to one another.Figure 4. Flat: The cylinders are arranged in two banks on opposite sides of the engine.
Let’s look at some key engine parts in more detail.
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.
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.
A piston is a cylindrical piece of metal that moves up and down inside the cylinder.
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. Many modern vehicles use more advance materials for piston rings. That’s one of the reasons why engines last longer and can go longer between oil changes.
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.
The crankshaft turns the piston’s up-and-down motion into circular motion just like a crank on a jack-in-the-box does.
The sump surrounds the crankshaft. It contains some amount of oil, which collects in the bottom of the sump (the oil pan).
Next, we’ll learn what can go wrong with engines.
Sep 18, 2019 at 4:56 pm #39687Car engines can have all sorts of problems, whether fuel related or battery related. ZERO CREATIVES/GETTY IMAGES
So you go out one morning and your engine will turn over but it won’t start. What could be wrong? Now that you know how an engine works, you can understand the basic things that can keep an engine from running.
Three fundamental things can happen: a bad fuel mix, lack of compression or lack of spark. Beyond that, thousands of minor things can create problems, but these are the “big three.” Based on the simple engine we have been discussing, here is a quick rundown on how these problems affect your engine:
A bad fuel mix can occur in several ways:
- You are out of gas, so the engine is getting air but no fuel.
- The air intake might be clogged, so there is fuel but not enough air.
- The fuel system might be supplying too much or too little fuel to the mix, meaning that combustion does not occur properly.
- There might be an impurity in the fuel (like water in your gas tank) that prevents the fuel from burning.
Lack of compression: If the charge of air and fuel cannot be compressed properly, the combustion process will not work like it should. Lack of compression might occur for these reasons:
- Your piston rings are worn (allowing the air/fuel mix to leak past the piston during compression).
- The intake or exhaust valves are not sealing properly, again allowing a leak during compression.
- There is a hole in the cylinder.
The most common “hole” in a cylinder occurs where the top of the cylinder (holding the valves and spark plug and also known as the cylinder head) attaches to the cylinder itself. Generally, the cylinder and the cylinder head bolt together with a thin gasket pressed between them to ensure a good seal. If the gasket breaks down, small holes develop between the cylinder and the cylinder head, and these holes cause leaks.
Lack of spark: The spark might be nonexistent or weak for several reasons:
- If your sparkplug or the wire leading to it is worn out, the spark will be weak.
- If the wire is cut or missing, or if the system that sends a spark down the wire is not working properly, there will be no spark.
- If the spark occurs either too early or too late in the cycle (i.e. if the ignition timing is off), the fuel will not ignite at the right time.
Many other things can go wrong. For example:
- If the battery is dead, you cannot turn over the engine to start it.
- If the bearings that allow the crankshaft to turn freely are worn out, the crankshaft cannot turn so the engine cannot run.
- If the valves do not open and close at the right time or at all, air cannot get in and exhaust cannot get out, so the engine cannot run.
- If you run out of oil, the piston cannot move up and down freely in the cylinder, and the engine will seize.
In a properly running engine, all of these factors are working fine. Perfection is not required to make an engine run, but you’ll probably notice when things are less than perfect.
As you can see, an engine has a number of systems that help it do its job of converting fuel into motion. We’ll look at the different subsystems used in engines in the next few sections.
Sep 18, 2019 at 4:58 pm #39688Figure 5. The camshaft
Most engine subsystems can be implemented using different technologies, and better technologies can improve the performance of the engine. Let’s look at all of the different subsystems used in modern engines, beginning with the valve train.
The valve train consists of the valves and a mechanism that opens and closes them. The opening and closing system is called a camshaft. The camshaft has lobes on it that move the valves up and down, as shown in Figure 5.
Most modern engines have what are called overhead cams. This means that the camshaft is located above the valves, as shown in Figure 5. The cams on the shaft activate the valves directly or through a very short linkage. Older engines used a camshaft located in the sump near the crankshaft.
A timing belt or timing chain links the crankshaft to the camshaft so that the valves are in sync with the pistons. The camshaft is geared to turn at one-half the rate of the crankshaft. Many high-performance engines have four valves per cylinder (two for intake, two for exhaust), and this arrangement requires two camshafts per bank of cylinders, hence the phrase “dual overhead cams.”
This content is not compatible on this device.Figure 6. The ignition systemHOWSTUFFWORKS
The ignition system (Figure 6) produces a high-voltage electrical charge and transmits it to the spark plugs via ignition wires. The charge first flows to a distributor, which you can easily find under the hood of most cars. The distributor has one wire going in the center and four, six or eight wires (depending on the number of cylinders) coming out of it. These ignition wires send the charge to each spark plug. The engine is timed so that only one cylinder receives a spark from the distributor at a time. This approach provides maximum smoothness.
We’ll look at how your car’s engine starts, cools and circulates air in the next section.
Sep 18, 2019 at 5:03 pm #39689
Engine Cooling, Air-intake and Starting SystemsThis diagram shows details of how a cooling system and the plumbing is connected.
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 pre-1999 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.
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 and modern fuel-efficient 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.
Since the turbocharger is reusing hot exhaust to spin the turbine and compress the air, it increases the power from smaller engines. So a fuel-sipping four-cylinder can see horsepower that you might expect a six-cylinder engine to put out while getting 10 to 30 percent better fuel economy.
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).
Sep 18, 2019 at 5:05 pm #39690
Engine Lubrication, Fuel, Exhaust and Electrical SystemsThe exhaust system of your car includes the exhaust pipe and the muffler. MARIN TOMAS/GETTY IMAGES
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 modern vehicles in two common ways: port fuel injection and direct fuel injection.
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). Older vehicles were carbureted, where gas and air were mixed by a carburetor as the air flowed into the engine.
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 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.
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