The Internal Combustion Engine

What is an internal combustion engine? It is responsible for turning an air/fuel mixture into motion. It does this by burning the mixture inside the engine to utilize the power output of the combustion explosion. There are different kinds of internal combustion engines like diesel or gas turbine. However, for the purpose of this article I am talking about ones utilizing gasoline as its primary fuel source.

In order to understand how an engine works, a look at engine makeup is required.


The block is the foundation of the engine. It is upon the block which everything else is mounted. Most blocks are made of either cast iron or aluminum. Most automobiles utilize 4, 5, 6, or 8 cylinder engines. There are many different configurations. Below are examples of different types of engine blocks.

Inline Four Cylinder

Inline Six Cylinder


Eight Cylinder

These are just a few examples. Other combinations exist, such as horizontally opposed (aka: pancake or boxer) four and six cylinder, V4, V10, W6, W8, W10, and many more. The examples shown should help you to understand the different combinations which are most typically found in vehicles today. At the base of the block reside the main bearing caps (or “main caps” or just “caps”). These caps provide the holding force for the crankshaft. The caps can be held to the block with two, four, or six bolts (or studs). The terms “two bolt main”, “four bolt main”, or “six bolt main” refer directly to the main caps and bolt configuration.


The piston’s only job is to fill the hole which is the cylinder. It is usually made out of an aluminum alloy, and can be cast or forged. Cast aluminum pistons come in several different types. Among them are ones with high silicon content known as hypereutectic.  Hypereutectic pistons are very thermally stable and can be machined for tighter tolerances. Forged pistons are made out of a solid piece of billet aluminum, but are machined down to its usable form.  Below is a picture of typical pistons.

Piston Rings:

The primary purpose of piston rings (or just “rings”) is to seal the piston within the cylinder. A secondary purpose of the rings is to control oil going into the combustion chamber or clear the cylinder walls of oil during the piston’s downward travel. Piston rings ride in grooves, called ring lands, in the sides of the pistons. In a gasoline engine there are typically four piston rings riding in three lands: two compression rings and two oil control rings separated by a spacer. Rings are made out of many different materials. For general production engines, cast iron is the material of choice. For aftermarket applications, such material as plasma moly (molybdenum) is used as a facing material which helps the rings to quickly “seat” (or seal) to the cylinder wall. Seating occurs during the break-in period. If a ring does not seat correctly, excess oil can enter the cylinder and a loss of cylinder pressure during the combustion cycle can occur. Another term which is used with rings is “end gap”. This refers to the distance or space between the ends of the rings as it sits in the cylinders. This is an important measurement, because if the rings do not have enough end gap when they heat up in the cylinder, they can actually cause the piston to seize within the cylinder. This will stop the engine until the ring cools down, but will happen the next time it gets too hot as well. There are rings which are made to be gapless. The regular way manufacturers can do this is by overlapping or stepping the ends of the ring so the gap is eliminated.

Connecting Rod:

The connecting rod does exactly as the name implies, it connects the piston to the crankshaft (which will be discussed next). Connecting rods (or just “rods”) can be made of many different materials and manufactured in many different ways. The most common manufacturing substance for a rod is cast iron. Iron is used due to it being inexpensive to work with, its ability to stand up to a lot of abuse, and for its longevity. Forged steel is another metal commonly used, but usually only in the aftermarket. It is stronger than iron, stands up to a lot of abuse, and also lasts a long time. The downside to forged steel is the cost. Aluminum alloy is usually reserved for racing engines because it tends to get stress fractures after what would be considered a relatively short amount of use.  Titanium alloy is another metal used, but very few production engines utilize the metal. The cost and manufacturing techniques involved with titanium is very prohibitive. Below is a picture of a typical connecting rod.


The crankshaft is a large piece of metal which turns the reciprocating motion of the piston, through the connecting rod, into rotating motion. This allows the power to be transferred through the drivetrain to the tires and thus we gain propulsion. Just like with connecting rods and pistons, crankshafts can be made of several different materials. Cast iron is most commonly used and machined to its finished shape. Forged steel is also used, but hardly ever in production vehicles as it is more expensive to manufacture. A crankshaft can be internally balanced, externally balanced, or a combination of the two. Each end of the crankshaft has its own balance and must be treated as such. Balance is important to a crankshaft due to harmonics and internal forces which occurs during operation. If not balanced properly, the parts connected to the crankshaft will vibrate and self-destruct over time. Here is an annotated image of a crankshaft:

Cylinder Head:

The cylinder head (or just “head”) has two main purposes. The first part is to seal the top of the cylinder to make it an enclosed space. The second, is to direct the air flow in and the exhaust flow out of the cylinder. Heads are made up of several components other than the head itself: valves, valve springs, retainers, keys (or locks), valve guides, valve seats, and spring seats. Valves are located within the head on all modern production engines and allow for both of these actions to occur. The valve rides within sleeve called a valve guide. The valve guide can be made of any of several different metals. The valve guide metal is chosen for its application and wear characteristics.  For most four stroke (4-cycle) engines, there are two valves per cylinder: one intake and one exhaust. Both are actuated by a camshaft (which will be talked about later as part of the valve train). Multi-valve engines are becoming more popular in production engines, which entail using more than two valves per cylinder.  Three (2-intake/1-exhaust), four (2-i/2-e), and even five (3-i/2-e) valves per cylinder can be found in modern engines. Basically, more valves equates to better flow if done correctly. Porting is a term which is used to describe the act of removing material from the head to encourage better flow through the head, into and out of the engine. Flow is measured in cubic feet per minute (CFM). Usually, the more flow, the more efficient an engine will run. Flow is measured on a flow bench, where both the intake and exhaust ports are measured separately. As a rule of thumb, if you double the number of CFM going into your engine, this is the approximate HP output the engine should be capable of at the crankshaft (ie: 300cfm = ~600hp). Heads are usually made of either cast iron or aluminum. Manufacturers are moving towards using more aluminum castings in the production of their engines as it saves weight and is easier to work with than is cast iron. Aluminum will warp more easily than cast iron if overheated, so therefor is used less frequently in engines which will observe a large amount of abuse (ie: trucks, cabs). Heads are sealed to blocks using head gaskets. Head gaskets allow similar or dissimilar materials (cast iron blocks/aluminum heads) the ability expand at different rates while still maintaining a seal. If a head warps, the head gasket can usually no longer seal the head/block together, allowing either gasses to escape during the combustion cycle, coolant to enter into the combustion chamber, or both.

Spark Plug:

The spark plug provides the ignition source for the air fuel mixture to allow it to combust. The spark plug sits within threads located in the head. When the terms platinum or iridium are used in conjunction with spark plugs, it refers to what the center electrode is made out of. The more rare the material, the more expensive the spark plug, but usually the longer the spark plug will last. Most production vehicles have spark plugs which do not be need replaced for 100k miles or beyond. There are many different configurations for spark plugs.

Timing Chain/Gears (or Timing Set):

In OHV engines, the timing chain and gears provide the function of keeping the valve events happening at the right time. The smaller of the two gears is placed on the crankshaft. The larger of the two is placed on the camshaft. Both are connected with the timing chain. The larger gear has twice the circumference of the small one. This allows the camshaft to turn at one halve the speed of the crankshaft. It is located at the front of the engine behind the timing cover, which is behind the water pump. In high performance applications, the set can be replaced by gears and timing belt or gear drive.

Valve Train:

The valve train refers to the system which actuates the valves in the head. This usually includes the valves and supporting components as well. Below is a breakout of those components and what they do:

  • Camshaft (or just “cam” or “bumpstick”): As the name implies, the cam is a long shaft which has a row of actuator cams (or lobes) along its surface. These lobes provide precise actuation (or lift) at proper intervals to actuate the valves allowing the air/fuel to move into the cylinder and the exhaust gasses out of the cylinder. The cam runs at precisely ½ the speed of the crankshaft. Because there is a lot of information to relay about a camshaft, this is a just a basic introduction.

  • Lifter: The lifter rides upon the camshaft lobe. It provides a means by which the cam motion can be transmitted off of the cam lobe to the valve. There are two basic lifter types: hydraulic and solid. Of these two basic lifter types, there are two different variations: flat tappet and roller. Each has its place, though roller types seem to be used more often these days. (Think of flat tappet as old school and roller being the new school lines of thought.) Hydraulic lifters are most commonly used in production engines, while solid lifters are used in racing applications. This is due to the ease of use and maintenance with hydraulic lifters: solid lifters require setting lash (described in “How a Camshaft Works”) at regular intervals to maintain peak performance. Solid lifters can attain higher performance levels while being more consistent and stable, especially at high lift levels and engine speeds. They also produce more noise. Hydraulic lifters are used quite often for performance applications where the vehicle will pull double duty on the street. Roller lifters are used more commonly now in production engines because they allow higher lift with great ramp rates (also explained in “How a Camshaft Works”) with less frictional losses imposed upon the valve train. This allows the engine to produce more power at a lesser cost. The hydraulic lifter allows oil to build up within the body of it, so as to provide the cushioning effect for the valve train. Oil is also transmitted through it to the push rod. Flat tappet lifters turn in their bores (located in the block) which allow them to achieve even wear across the flat surface at the base of the lifter. When utilized and broken in on a cam, the flat tappet can no longer be used with any other cam or cam lobe. When the break-in process occurs, a sympathetic wear pattern exists between the cam lobe and the lifter. If used on a different lobe after this point, destruction of both the lifter and lobe will occur in a short order. Roller lifters do not suffer from this issue and can be reused if in reasonable condition.

Roller Lifters

Flat Tappet Lifters

  • Push Rod: The push rod is a small, pencil sized piece of metal set between the lifter and the rocker arm (described next). It transmits the actuation provided by the cam through the lifter up to the rocker arm. It can be made several different metals, but usually are made out of either steel, tempered steel, or carbon fiber. The carbon fiber will have metal balls attached at its ends. Application dictates how long or thick the push rods are. A mechanic can fine tune the valve train using different lengths of push rods.

  • Rocker Arms: Rocker arms (or just “rocker”) are used to change the direction of the up motion of the push rods, to the down motion needed to actuate the valves (it acts as a fulcrum). Rocker arms usually have a ratio associated with them to allow a magnification of the lift measurement at the camshaft to be turned into a greater total lift at the valve itself. (If this doesn’t make sense, it will be covered in greater detail later in the “How a Camshaft Works” article). Rocker arms come in many different variants, including (but not limited to) stamped steel, machined steel, cast iron, aluminum, full roller, roller tip, etc. These combinations and uses will be described in a later write-up. The rocker arm is where the adjustment for the valve train is made. This adjustment allows for the rocker tip to remain in contact at all time with the valve tip (in the case of a hydraulic lifter), or for a predetermined amount of “lash” to be allowed (in the case of a solid lifter). Most production engines (overhead valve) pushrods in use today have the adjustment already engineered into the rocker arms and associated valve train. These types do not readily have the ability to be adjusted.

  • Lock (or key): A lock is used to keep the valve and valve spring together as a single unit. It is typically made from machined steel and as of late is being made out of titanium for weight savings. The angle of the retainer, the lock, and the pressure of the valve spring, forces the key into the tip of the valve, capturing it and keeping everything together. At the tip of the valve there is a groove which corresponds to a ridge on the inside of the key. It is a common myth this actually keeps the key from coming off (or letting go of the valve). It is in fact the pressure of the lock being forced into the valve which causes an interference that holds onto the valve stem. The groove just helps to locate the lock during assembly.
  • Retainer: The retainer keeps the valve spring located with the valve via the lock. It is usually made from either machined steel or titanium. Titanium is primarily used in aftermarket applications for weight savings. This additional weight savings allows for higher engine rpm to be achieved without the risk of valve float. Valve float occurs when the speed of the engine is great enough the valve is not given enough time to close completely before it is required to open again. This causes loss of combustion chamber pressure during the combustion cycle and thus hampers performance of the engine.
  • Valve Spring (or just “spring): The valve spring provides the force needed to close the valve after it has been opened. Springs are made from steel wire which has been formed as a spring to specific dimensions, depending on what is required for an engine combination. The formed spring is then heat treated to obtain the resilience required for an application. Springs can come as single, double, or even triple spring combinations. The double and triple combination springs are used in high performance applications where additional force is required to overcome the inertia of the valve during engine operation.
  • Spring Seat: A spring seat performs two required functions. It locates the base of the spring in a specific location in the head, and also provides a level of protection for the head so the spring does not dig into the softer aluminum of the head during engine operation.
  • Shim: The shim provide the engine builder the ability to fine tune the spring height and seat pressure of the valve spring.
  • Valve: The valve is a device which covers the ports in the head to seal the combustion chamber. It is made up of several different parts: the head, stem, face, and tip. Valves can come in many different sizes and materials. Materials can include steel, stainless steel, and titanium. Valves can have hollow stems to provide weight savings, as well as sodium filled exhaust valves to promote heat transference.

  • Valve Seat: Valve seats are a ring which is pressed into the combustion chamber of the head. It provides a hard surface for the valve to sit and seal against when closed. It is usually made of cast iron. It is the valve seat which is machined when the term “3 angle” or “5 angle” valve job. By “cutting” the valve seat at several different angles, the valve seat becomes radiused which can allow smoother and greater air flow into the cylinder during the intake cycle. One of the “cuts” made during the machining process will match a cut on the valve which promotes sealing of the chamber. A machinist will further “lap” the valve to seat interface to promote better sealing. Lapping is done by putting a compound on the two faces and turning the valve to produce a sympathetic wear pattern between the two.

Oil Pump:

As the name implies, pumps oil throughout the engine. Can be normal pressure/normal volume (stock), normal pressure/high volume, high pressure/normal volume, or high pressure/high volume. Oil pressure needs to be maintained during engine operation or oil starvation, damage, and eventually engine destruction will occur.


While many would not consider oil a “part” in the engine, it provides many different functions. The oil does the following: lubricates, cushions, cools, cleans, and reduces friction. Without oil, the engine would come quickly to a screeching halt.

Oil Pan:

Oil is stored here until it is pumped throughout the engine via the oil pump. It also catches oil as it returns to be pumped again. It also covers the crank case (where the crank shaft resides) which keeps dirt and grime out of the engine.

Now that you know the different parts of the engine, we’ll discuss how those parts interact to make the power needed to motivate your vehicle.

Most automobile engines go through a 4 stroke cycle (aka: four stroke or four cycle or Otto cycle … the terms can be used interchangeably). The 4 strokes are: Intake, Compression, Power (Combustion), and Exhaust (someone coined an easy phrase to remember this: suck, squeeze, bang, blow … that’s the way the engine goes!). There are other types of internal combustion engines which use different stroke cycles, such as the two-stroke and Miller cycle engines.

Intake Stroke:

The piston starts at top dead center (TDC) to begin the cycle. The intake valve opens up and the piston moves towards the bottom of the cylinder. This motion creates a vacuum which draws the air/fuel mixture into the cylinder. As the piston reaches the bottom of the cylinder or bottom dead center (BDC), the intake valve begins closing which creates a sealed chamber.

Compression Stroke:

The piston begins to move from BDC back up through the cylinder. As it does, it compresses the air/fuel mixture. Through this entire cycle, the valves remain closed.

Combustion or Power Stroke:

Near the end of the Compression stroke, a spark occurs which starts combustion in the cylinder. As combustion occurs in the cylinder, extreme pressure begins to fill the small space which is left as the piston reaches TDC. This slows and then forces the piston back down towards BDC. Just prior to the piston reaching BDC, the exhaust valve begins to open which allows the exhaust gases to escape. This siphons off the pressure from the cylinder.

Exhaust Stroke:

As the piston travels back towards the top, the exhaust
valve become fully open and releases the exhaust gases out through the exhaust system. The piston continues to travel upward expelling all of the exhaust gases. Near the end of the exhaust stroke, the intake valve begins opening to prepare for filling the cylinder during the intake stroke.

Cooling System:

Most cooling systems consist of a radiator, thermostat, water pump, hoses, fan, and coolant. Through paths in the block and in the heads, coolant flows to pull heat from the engine and sends it to the radiator where it is cooled off. The same fluid flows back through the engine to do the same thing over and over again. Temperature is regulated via the thermostat, which opens and closes to restrict and direct the coolant flow. Some heat needs to be built up and retained within the engine during operation so it will work more efficiently. If an engine gets too warm it will often experience detonation (or pre-ignition) where hotspots within the combustion chamber will cause the air fuel mixture to ignite prior to when the spark plug sparks. This also causes excess Nitrogen Oxides (NOX) to be produced due to excessive combustion temperatures. NOX is the basis of acid rain and is very hard on the lungs. If the engine is too cool, it will not get complete combustion. This causes un-burnt fuel to exit the tail pipe, reduces power output, and can burn out and/or plug your catalytic converter.

  • Radiator: The radiator is usually made out of aluminum due to its radiant properties and weight savings. It has tubes which coolant flows through set up in rows and separated by cooling fins. There are usually two “tanks” set up on either end of radiator. These tanks are connected to the ends of the tubes and provides an entrance and exit for coolant to settle while in transit through the radiator. Radiator hoses are used to direct the coolant out of the water pump, into the radiator, and back to the engine.
  • Water Pump: The water pump is the driving force behind the coolant flow. It us usually located at the front of the engine. It can be driven externally via a fan belt, or internally off of a timing belt or gear, depending on the type of engine it is. Some aftermarket types of water pumps are driven by an electric motor which frees up horsepower from the engine, allowing the engine to operate more efficiently. Most water pumps come with a “pee” or “weep” hole cast into the body of the pump. If the internal seals or bushings start to wear out, the coolant will start to “pee” (leak) from the hole. If the pump is not replaced in short order, the unit can fail which can cause catastrophic failure of the engine.
  • Thermostat: As stated, the thermostat regulates the flow of the coolant, as well as directs it. Here’s a great video showing the thermostat in operation:

Air Flow:

The engine needs the air/fuel mixture to properly go through the 4 strokes. Obviously, the more air flow you have, together with the right amount fuel, you can create a more powerful explosion inside the combustion chamber… meaning more power. So with that being said, what makes a turbocharger or supercharger so special?

To make a long story short, turbochargers and superchargers compress air BEFORE it enters the cylinder and once again after it is inside the cylinder. Remember, an engine without a Turbo or SC compresses the air AFTER it is already in the cylinder and only after. Ever heard of boost? Well the amount the turbo or SC compresses the air is your boost. The reason these devices compress the air before sending it into the cylinders is so more air will fit… you remember what that does right? The amount of pressurization of air is what determines how much power the turbo or SC can add to your engine.

Fuel System: 

Basically, fuel is pumped from the fuel tank to your engine so it can be mixed with air and run the engine. There are a several different ways to accomplish this task. The three most common ways are Carburetion, Port Fuel Injection, and Direct Fuel Injection.

In a carbureted engine, fuel is drawn into the air as the air comes through the venturis of the carburetor. Basically, air is met by fuel immediately after coming through the air intake. The air/fuel mixture then flows into the different cylinders.

In fuel injected engines, fuel is metered by use of fuel injectors to provide each cylinder with the correct ratio of air to fuel for the engine to run efficiently. Port fuel injection engines have a unit which basically sits where a carburetor sits on top of the intake manifold. The correct amount of fuel is sprayed into the incoming air stream to create the air/fuel mixture. This is commonly referred to a “wet” intake tract (the same as a carburetor). Port fuel injection places the fuel injector just behind the valve and releases fuel just as the valve opens during the intake stroke. This requires greater fuel pressure to ensure proper fuel delivery. Direct fuel injection sprays fuel directly into the combustion chamber during the compression stroke. This process allows for greater compression to be utilized which produces more power output for the same amount of air/fuel used. This allows for better fuel mileage as well. Direct injection is used in newer gasoline engines as well as having been used in diesel engines. Direct injection requires even greater fuel pressure than the port injection so as to overcome the internal pressures incurred during the compression stroke. Port and Direct Fuel injection are considered “dry” intake tracts.

Exhaust System: 

The exhaust system consists of headers (exhaust manifolds), exhaust pipes, catalytic converters, and mufflers.

  • Exhaust Pipes: Exhaust pipes route hot exhaust gasses from the headers through the different components to the rear of the vehicle. These pipes are usually bent into a shape which allows them to conform to the underside of the vehicle while avoiding moving parts such as the rear axle. Several different methods are used to produce the bends in the pipes, which include crimp and mandrel bends. Crimp bends pinch the metal of the exhaust pipe to form the bends. This is a very cost effective way to create exhaust pipes at the cost of restricting the flow. Typically, the easier the exhaust flow is allow to articulate through the exhaust system, the less back-pressure is observed. Excessive amounts of back-pressure can cause loss of engine output. Mandrel bends are formed using a device to bend the pipe while allowing it to maintain its shape and size. This type of bend allows for easier transitions of the exhaust flow, which allows for less back pressure. This type of bend requires more expensive equipment to perform the bend, as well as more time to create them. More vehicle manufacturers are more readily using mandrel bends as standard exhaust systems. This is because of the ever tightening gas mileage restrictions for vehicles. In this case, every little bit of fuel economy a manufacturer can squeeze from their vehicle the better. Exhaust systems in general are usually made from soft steel or stainless steel. Soft steel is easy to form and bend and is cheap to produce. It has the propensity to rust over time, which will cause failure and need replacement. Stainless steel is harder to form and more expensive to make, but will generally resist corrosion, which allows it to last much longer than soft steel. This is very important in areas where salt and brine solutions are used in the winter months to clear ice covered roadways.

  • Muffler: The muffler quiets the noise created from the combustion of the air/fuel mixture. Most cars from the factory have noise regulations which limit the amount of noise a car can make. Mufflers can reduce the sound in one of two ways: through stuffing the chambers within the muffler with sound deadening materials, or through noise cancelling techniques. Usually, non-combustive fiberglass insulation is used as a filler within the muffler. It absorbs the noise as the exhaust gasses pass through the muffler. This is a very cost effective way to reduce the noise produced, but can create back pressure within the exhaust system. Noise cancelling techniques involve bouncing the sound off of different reflectors within the muffler, trapping the noise inside of the muffler itself. It does this while creating a minimal amount of back pressure on the exhaust system. This is being employed more and more by auto manufacturers as the cost to make this type of muffler is reduced.
  • Catalytic Converter (or cat): The catalytic converter is responsible for reducing the amount of harmful gasses released into the air. It also helps burn off unused fuel before it exits the exhaust system. It does this through the use of catalysts which are plated onto a honeycomb of metal which the exhaust flows through. Different catalyst materials are used to create the reactions, such as platinum. Since platinum is such an expensive element, thieves target catalytic converters by cutting them off of the vehicle and then selling them to metal recyclers at a very tidy profit. Catalytic converters come in either two- or three-way configurations, depending on what types of gasses you want to decrease. A two-way converter controls hydro-carbons (HC – unspent fuel) and carbon-monoxide (CO). A three-way converter also controls nitrogen-oxides (No2 – not to be confused with nitrous-oxide or N2O). As the gas flows over the catalyst, the gas molecules lose their chemical bonds, allowing them to reform into non-lethal gasses. NO2 is the basis of smog and acid rain. As the gas passes over the catalyst, it is fundamentally reformed into nitrogen (N2) and water (H2o), which are completely harmless.

Ignition System: 

The ignition system is responsible for providing the spark which ignites the air/fuel mixture within the combustion chamber. There are basically two different types of ignitions systems in use today, with many variants to confuse things. One system uses a distributor and one does not. A distributor style ignition system consists of a coil, spark plug wires, coil wire, spark plugs, distributor cap, rotor, and distributor. A distributorless system has multiple coils, spark plug wires, and sparkplugs.