Brief Engine History
In 1876, Nikolaus Otto (Germany) along with Gottlieb Daimler (Germany) and Wilhelm Maybach (Germany) developed and patented the 4 cycle internal combustion engine. In 1886 Karl Benz (Germany) developed and patented the first automobile. Most of the concepts that were put into the original automobile are still used today. We might have changed how fuel is delivered/managed, or how spark is created, but the four stroke cycle hasn’t changed in more than a century. Other engines that we still use today are the diesel engine, (Rudolf Diesel of Germany) the rotary engine (Felix Wankel of Germany) and the 2-stroke engine. (Dugald Clerk of Scotland)
The 4-stroke Cycle
A car/truck is a machine designed to get people or items from point A to point B. They way the vast majority of cars do this is still done using the same basic principles as the original automobile. An engine relies on a controlled burn (not an explosion) taking place inside the combustion chamber to push a piston down and turn the crankshaft to push the car/truck forward. For this to happen the engine needs fuel, compression/air (oxygen), and a spark to ignite the mixture. The four strokes that make that happen are intake, compression, power and exhaust. The old cliche people seem to remember is suck, squeeze, bang, blow. Out of the four strokes, only one of them makes useable power.
- Intake Valve: open
- Exhaust Valve: closed
The piston is moving down, this creates a low pressure area in the cylinder. Atmospheric pressure pushes the air/fuel mixture into the cylinder to fill the void.
- Intake Valve: closed
- Exhaust Valve: closed
The piston is moving up, this compresses the air/fuel mixture. Typical compression pressures are between 100-250psi.
- Intake Valve: closed
- Exhaust Valve: closed
The spark plug ignites the air/fuel mixture causing a controlled burn and an extremely high pressure area in the combustion chamber that forces the piston down and turns the crankshaft.
- Intake Valve: closed
- Exhaust Valve: open
The piston moves back up and forces the used up air out of the cylinder.
This cycle repeats itself on all cylinders until the engine is shut off. Automotive engines have more than one cylinder. That is because it is difficult to get one piston to complete a full cycle under its own power. Most engines today have at least 4 cylinders. On a 4 cylinder engine, each piston is on a different stroke at all times. One cylinder is always pushing the other 3 (as well as the vehicle) through their cycles. Cylinders can be arranged inline, in a V type, in a boxster/flat style and in a W type. Each cylinder needs to make at least enough power so the engine will rotate to the next cylinders power stroke. The amount of power created on each powerstroke depends on how much air and fuel are allowed into the cylinder. This is controlled by the fuel injectors and throttle body or by a carburetor.
- Intake Valve: partially open
- Exhaust Valve: partially open
You may also hear of people refer to the “Overlap Stroke.” Overlap is not really a stroke but it is important for an engine to run properly, as well as controlling emissions. Overlap is the short period of time when both the intake and the exhaust valves are open between exhaust and intake stroke. This allows a small amount of air to pass through the cylinder and flush out any used up exhaust gasses that may not have left the cylinder. This process is called scavenging.
Terms to Know
- TDC: Top Dead Centre, it means when the piston is at its upper most point of travel
- BDC: Bottom Dead Centre, it means when the piston is at its lower most point of travel
- Short Block: The engine block with a crankshaft, pistons and corresponding bearings
- Long Block: The engine block with the head bolted down with all pistons, rods, crankshaft, bearings, camshaft and valvetrain. No manifolds, belts or other accessories.
Here is a good animation of an engine running:
Here is a live shot from inside a combustion chamber:
Pieces of an Engine
The engine block is the main lower structure of the engine. It houses the crankshaft, cylinders, water jackets and oil galleries. They can be cast iron or aluminium, but aluminium blocks have sleeves in them for the cylinder walls. Engine blocks need to be strong, they can either be made from a sand mold (stronger) or a styrofoam mold. (weaker) If the outer metal of the block looks like styrofoam, with small circles in the finish, it was made in a styrofoam mold. Some blocks are of a 2 piece design. The lower portion, called a bedplate, is basically all the crankshaft main bearing caps built into one piece. This setup is typically stronger but they may not have a gasket. If oil leaks from between the bedplate and the main block it an be very costly to fix.
A crankshafts main purpose is to convert the linear motion of the pistons and connecting rods into rotational motion. Its other purpose is to splash oil onto the cylinder walls for lubrication. They may also have a reluctor wheel for the crankshaft position sensor. Crankshafts are generally cast iron or forged steel. They also have counterweights opposing the connecting rod journals to balance the the rotational motion of the connecting rods. A journal is any place that has a bearing that allows movement between the two devices. Journals and bearings need a constant supply of pressurized oil, both for lubrication and cooling. Inline engines have one connecting rod journal per cylinder but many V-type engines have 2 cylinders that share a journal. Crankshafts need 2 types of bearings, main bearings and connecting rod bearings. Main bearings support the crankshaft journals in the engine block and allows the crankshaft to rotate freely. Most engines have one main bearing that has flanges on either side. This is done to better control horizontal crankshaft movement or endplay. The connecting rod bearings sit between the connecting rods and the crankshaft to allow the connecting rods to rotate the crankshaft. Since the connecting rods are moving when the engine is running, holes are drilled in the crankshaft to deliver oil from the main bearings to the connecting rod bearings. Since the main bearings stay in the same place, they get oil first, the oil then travels through the holes in the crankshaft to the connecting rod bearings.
The pistons must be able to take the abuse from combustion but they also need to be as light as possible so most are made out of aluminium. Pistons can be cast, forged or hypereutectic*. The pistons are attached to the top or small end of the connecting rod by a wrist pin or piston pin. Wrist pins can be press fit or they can be held in by a clip. The hole for the wrist pin may be not be centred in the piston. This is to control piston slap and wear on the major thrust surface which will be discussed later on. Pistons can be installed backwards so be careful. Pistons have grooves in the side to keep the piston rings in place. This area is called the ringlands. Most pistons will have 3 piston rings. The area below the wrist pin is called the skirt. Most cars have what is called a slipper skirt, this design has extra skirting only on the thrust surfaces and not on the sides. This is done to keep the pistons as light as possible and provide clearance for the crankshaft counterweights when the piston is at BDC. A full skirt, which has skirting all the way around the piston, is usually found on more heavy duty applications. Aluminium expands at a different rate than cast iron. To accommodate this, many pistons are what is called “cam ground.” Cam ground pistons are slightly oval. The longer ends ride on the thrust surfaces so they have a tight fit at all temperatures, and as the piston warms up the skinnier sides start to expand out towards the cylinder walls. Some pistons are partially or fully molybdenum coated. (black coating) This reduces friction and scuffing.
*Hypereutectic pistons have silicon mixed in the aluminium, they tend not to expand as much so the engine can have much tighter clearances from the piston to the cylinder walls even while the engine is cold.
There are two main types of piston rings. Compression rings are used to seal compression and combustion inside the cylinder. During compression and powerstroke, a small amount of pressure is allowed into the area between the piston and the ring. This pressure pushes the piston ring out against the cylinder wall, creating a tight seal. Oil rings serve a different purpose. Oil rings sit bellow the compression rings and are designed to scrape oil off the cylinder walls, but not 100% of it. A small amount of oil is allowed to get by to lubricate the upper cylinder wall. These rings are usually a 3 piece ring, the middle ring being wavy. The groove that oil rings sit in usually have holes in them so oil can pass through the ring, through the piston and back to the oil pan. A middle ring may be 50% oil ring and 50% compression ring. Both kinds of rings are also designed to transfer heat from the piston to the cylinder walls.
The combustion chamber is the area above the piston when it is at TDC. This is where the valves and spark plug are located. It is also where combustion begins. The size and shape of the combustion chamber are both important factors in an engines performance and emission levels. To get the most efficiency out of a combustion chamber, the spark plug should be at the centre. This allows the flame front to contact the middle of the piston first and travel outwards evenly in all directions across the piston. One thing of note is air/fuel mixture turbulence. The more the air moves around in the cylinder, the better the air and fuel will mix, causing more complete combustion. Some engines have a place or multiple places in the cylinder head that the piston almost touches when it is at TDC. This causes the air that was directly under this area to be pushed out of the way very quickly, creating air turbulence. These areas are called quench areas.
Connecting rods transfer movement from the piston to the crankshaft journals. The “small end” rides on the piston pin and the “big end” rides on a bearing on a crankshaft con rod journal. Connecting rods take a lot of abuse so they must be strong, but at the same time they must be made as light as possible. Cast iron or steel con rods are widely used. Most OEM pistons are of an I-beam design for strength but they can also be H-beam or other designs. Some con rods have a hole on the big end that goes right through to the small end. These are called rifle barrelled con rods. There is a hole in the upper big end con rod bearing that lines up with this hole. The hole supplies oil to the con rod small end for lubrication and cooling.
Main Bearings: Main bearings sit between the crankshaft main journals and the block. They centre the crankshaft in the block and allow the crankshaft to spin freely as well as in some cases control crankshaft endplay.
Connecting Rod Bearings: Con rod bearings sit between the crankshaft con rod journals and the con rods. They take quite a bit of abuse, all the engines firing pulses must go through these bearings before they get to the crankshaft.
In a perfect world, two metal surfaces would never contact each other inside the engine. Oil is pumped between the bearing and the journal to act as a cushion between the two. If the oil could keep the 2 fully separated 100% of the time, we wouldn’t need bearings. If there was zero metal to metal contact in the engine, there would be no engine wear. In the real world, we need something to take the abuse of minor metal to metal contact. Most engine bearings are made up of three metal layers. The first layer, in contact with a journal is babbitt. Babbitt is soft and provides a bit of cushion. The next layer can be copper or lead. All of this is on a steel backing. Engine bearings are made at a little wider of a curve than the casting they fit into. This is called bearing spread and is used to help keep the bearing in place during assembly. Bearings may or may not have a locating tab, these make sure the bearing is centred in its casting. Bearings may also have an oil groove in them for extra oil flow for lubrication or cooling. Once he bearing is in its bore you may notice that the ends of the bearing stick out a little bit from their castings. This is called bearing crush. When the 2 castings are torqued down together, the bearing is pressed up against its outer castings to help heat transfer as well as keep the bearing from spinning in its casting.
Cylinder heads house the camshaft, valves, valve springs, valve seats, valve guides, rocker arms (if equipped) spark plugs, and water jackets. Older heads were cast iron but most modern heads are made out of aluminium to save weight. In most engine setups the head is where combustion starts. The combustion chamber is something that engineers like to change quite a bit because it is such a huge factor in the overall performance of the engine. Scroll down for more on combustion chambers. Special notes: When removing a cylinder head, remove the head bolts in the reverse order as tightening to avoid cracking or warping the head. Always use new head bolts unless otherwise specified.
Spark plugs provide ignition to the air/fuel mixture to start the combustion process. The spark plug is supplied with 15,000+ volts (newer systems are much higher), to get an electrical arc to jump across the air gap and ignite the mixture. For more on spark plugs check out our ignition systems page.
The head gasket sits between the block and the head. It seals and separates combustion, crankcase gasses, coolant and oil. Head gaskets are one time use because they crush when the head is tightened down. Some head gaskets come in different thicknesses, make sure the gasket you install is the correct thickness for the application. Most modern head gaskets are metal, even when they are removed it is difficult to tell where the problem was.
Camshafts and Camshaft Timing
Camshafts open (not close) the valves. The camshaft(s) is driven at half the speed of the crankshaft by a belt, chain or meshed gears. The camshaft must be timed with the crankshaft to maintain proper valve timing. On some engines, if the timing belt breaks or the engine is not properly timed, the pistons can hit the valves causing serious engine damage. Usually the exhaust valve gets hit because on exhaust stroke the piston is on its way up as the valve is open. Engines that can have piston to valve contact are called interference engines. Valve springs keep the valves closed until the cam lobe opens the valve. Camshafts may or may not have its own bearings. Camshafts can be in-block or overhead cams. In-block cams are located in the engine block and require pushrods and rockers to open the valves. Overhead cams are above the cylinder and may or may not have rockers. If their is no rockers, it is called a cam-on-bucket or cam-on-lifter setup. In all the diagrams and explanations around the web, they always make it seem like the 4 strokes all take 180° of crankshaft rotation. This is not the case. The cam action opening the valves are what really determine the length of each stroke. Intake and exhaust strokes tend to be much longer than 180° and compression and power tend to be much shorter. Camshaft duration is how many degrees of CRANKSHAFT rotation the valve is open. This does not have to be the same for the intake and exhaust valves. Lift is how deep the valve opens. The deeper the valve opens the more the valve is out of the way and the easier the air or exhaust gasses can get around the valve. The restriction the valve causes by forcing intake air or exhaust gasses to go around the valve is called shouldering.
Intake lead is the number of degrees of crankshaft rotation that the intake valve starts to open before the piston gets to TDC. This determines when overlap starts as well as affecting idle quality. Engineers use intake lead as a way to get as much air into the cylinder as possible, but if there is too much intake lead the engine will not idle well. At low RPM the air doesn’t have enough momentum to enter the cylinder while the piston is still moving up and the air will stop in the intake port. That means that the intake valve being open sooner partially stops airflow into the cylinder, causing the engine to stumble. When the engine gets into high RPM, air momentum overcomes the piston and air enters the cylinder. Some of this air is used for scavenging and some is held in the cylinder through the intake and compression stroke and used for combustion. On a race car idle quality is not a concern, this is why you see these big V8 drag cars that can barely idle but they absolutely scream down the track.
Intake lag is the number of degrees of crankshaft rotation that the intake valve stays open after the piston has passed BTC. Just like intake lead, intake lag is used to get as much air into the cylinder as possible. Intake lag cuts into the compression stroke. Many people think this is a bad thing that will hurt engine performance but it doesn’t, if the cam is designed correctly. Near the end of the intake stroke, air has loads of momentum in the intake port/runner. Also, the piston is at BDC and the air that is already in the cylinder can compress so intake lag is not as crucial to idle quality as intake lead is. The airs momentum on intake lag can overcome the piston moving upwards much better than air during intake lead so engineers tend to have more intake lag than lead to get as much air into the cylinder as possible.
Exhaust lead is the number of degrees of crankshaft rotation that the exhaust valve starts to open before the piston gets to BDC. Exhaust lead uses the expanding gasses in the cylinder to start purging the cylinder of used up exhaust fumes. The power stroke ends the moment the exhaust valve starts to open. It is much easier for the still expanding gasses to exit the cylinder through the exhaust port than it is to push the piston down and move the car. It is not a serious loss to end the power stroke early. By the time the piston gets near BDC the piston has almost no leverage over the crankshaft anyway.
Exhaust lag is the number of degrees of crankshaft rotation that the exhaust valve stays open after the piston has passed TDC. This is done so that overlap can take place. The longer the exhaust valve stays open into the intake stroke, the more air will flow right through the cylinder.
Variable Valve Timing
Variable valve timing allows the engines camshaft timing to change as engine speeds/loads change. Some setups will alter the intake valve timing only, others will change the intake and exhaust valve timing and others will change the intake and exhaust valve timing as well as valve lift. Typically the intake valve timing will be retarded or slightly behind normal at low engine speeds and advanced at moderate to high engine speeds. (see intake lead to find out why) Manufacturers have used several ways to adjust valve timing. One way is a cam phaser. A cam phaser sits on the driven end of the cam. When the engine wants the cam to advance it sends a signal to the phaser and the phaser uses an electromagnet or oil pressure to push the cam slightly ahead of the drive pulley/sprocket. Another way is to drive the exhaust cam with a fixed timing belt, and have the intake cam be driven by the exhaust cam via a chain. The chain tensioner can move the slack in the chain to above the sprockets, or below. Another way still is to have a completely different cam lobe for different engine operating conditions.
Every four stroke engine has at least one intake and one exhaust valve. The intake valve allows the air/fuel mixture to enter the cylinder on intake stroke and the exhaust valve allows the used up exhaust gasses to leave on exhaust stroke. They also must seal in compression and combustion against the valve seats on compression and powerstroke. This is done by using high tension valve springs to keep the valves closed. Valves are usually made of steel or stainless steel. Valves must be made as light as possible to avoid valve float* at high engine RPM. The intake valve is always larger than the exhaust valve, (except VW/Audis 5v heads) this is because it is much harder to get air into the cylinder than it is to get the air out. On a naturally aspirated engine, the only thing pushing air into the cylinder is atmospheric pressure which is about 1bar (14.5psi). When the exhaust valve opens, some of the combustion pressure should still be present to get the exhaust moving out of the cylinder, followed by the piston moving up the cylinder, forcing the exhaust gasses out of the cylinder. Valves also need to be cooled, especially the exhaust valve. Normally valves cool themselves by transferring heat to the valve seats when the valve is closed. Sometimes that isn’t enough, so some valves are sodium filled to disperse heat better.
A multivalve engine is an engine that has more than 1 intake and 1 exhaust valve. The reasons manufactures do this is more valves equals more air flow and 4 smaller valves are lighter than 2 big valves. The more valves you have, the more air you can displace, resulting in higher possible RPM and HP. Smaller valves have a smaller valve head, air can get around 2 smaller valve heads easier than one big valve head. Also with this added airflow making higher RPM possible, valve weight becomes a factor. As RPM increase, the speed the valves are opening and closing becomes faster and faster. With big, heavier valves we would need much stronger valve springs, (which actually use up engine torque to compress), to prevent valve float. But with more, smaller, lighter valves, we get all the extra airflow without the need for heavy valve springs. Yes, that means their would be more valve springs in the cylinder head to compress, but in the end the power gains and extended RPM range from an engine with 4 small valves per cylinder will out perform an engine with 2 big valves per cylinder.
Valve springs hold the valves closed until the camshaft opens it. They also provide the pressure necessary for the valves to seal against the valve seats. They are held in place by the retainers. The retainers are held in place by the pressure of the valve spring and the keepers. The keepers are little pieces that fit into grooves on the valve stem. Valves springs must be stronger for high RPM applications to avoid valve float, but the stronger they are, the harder they are to compress which actually hurts engine performance. Some engines have more than one valve spring per valve, the other valve springs are mainly to control vibrations.
The purpose of hydraulic lifters is to take up play in the valvetrain. As engines warm up, all metal components expand, if this was not compensated for, the valves could stay open a bit when they should be closed. Hydraulic lifters have hole in them that oil pressure is supplied to when the valve is closed. As the cam starts to push the lifter into its bore, that hole is sealed off and the lifter becomes solid. The lifter pumps up to take up space when it needs to and bleeds off oil when it needs to. Without oil pressure lifters are very soft and do not transmit the camshafts force to the valve or pushrod. Lifters normally ride on the camshaft, so in a cam-in-block setup they are located in the block between the camshaft and the push rods, and on a cam-on-bucket setup they are between the cam and the valve. Older engines had manual valves that needed to be adjusted to a happy medium for both cold and warm operation. These engines tend to have a clicking sound that goes away as the engine heats up. Most cam-on-rocker setups are manual valves but some manufactures have managed to incorporate hydraulic lifters into their setup.
From left to right: Valve spring, Retainer, Keepers, Exhaust Valve, Intake Valve, Hydraulic Lifter.
Valve guides centre the valve in their seat to maintain proper valve to valve seat contact. They are usually steel but they can be bronze. They take the abuse that the aluminium cylinder head couldn’t handle. They can be integral to the head and they can be a separate piece that is pressed in.
A valve seals job is to regulate (not completely seal) a small amount of oil from the cylinder head to the valve stem and guide. If their is not enough oil getting to the valve stem, the valve could stick in the guide. However if their is excessive oil on the stem, the oil may run down the stem and into the intake port in moderate quantities, causing the engine to use oil, blow blue smoke out the tailpipe as well as fail an emission test.
Balance shafts are used when the vibrations of the engine, (mostly from the crankshaft) are deemed unreasonable. They are mostly used on inline 4 cylinder engines in excess of 2.0L (122ci). They are basically a spinning offset weight. Their are usually 2 of them and they spin at twice the speed of the crankshaft. These must be timed to the crankshaft so they can properly cancel the crankshafts imbalance. They do however use up some of the engines torque to run.
Valve Float and Lifter Pump Up
Valve float is when the valves do not close when the camshaft intended. This happens at high engine RPM when the valve opens very quickly and the valve spring can’t overcome the valves momentum. The result is the valves staying open longer than the cam intended. This can be a problem because if the exhaust valve stays open too long, it runs the risk of getting smacked by the piston, also when the valve does close, the lifter will smack into the camshaft. The other problem is lifter pump up. When a valve starts to float, there is a momentary space in the valvetrain. The lifters job is to take up space in the valvetrain, when a valve starts floating, so does the lifter. If the exessive RPM continues the lifter may pump up to the point where a valve hangs open at all times. If this happens the engine will usually start to loose power and eventually shut down. If no damage was done to the valve, the engine just needs to sit for a while to let the lifters bleed down and it will start up again. If this happens the engine needs new/stronger valve springs if it is to continue running at those RPM.
Volumetric efficiency is the percentage of air that the engine takes in compared to the amount of air it could potentially take in. For example, a 2.0L engine will not displace 2.0L in one full engine cycle. (720° of crankshaft rotation) A typical engine will displace about 80% of its capacity. We can only get so much air in to the engine in such a small period of time with atmospheric pressure alone. So our 2.0L engine actually only displaces about 1.6L of air per full engine cycle. It is almost impossible, as well as expensive to design a naturally aspirated engine with 100% volumetric efficiency, while maintaining emission levels, fuel economy and idle quality. The only way to get around this is by forced induction. (turbochargers/superchargers) A typical turbo/supercharged engines volumetric efficiency is about 120%. The more boost is delivered to the engine the higher the volumetric efficiency.
Thrust surfaces are the areas on the cylinder walls perpendicular to the crankshaft centre-line or the wrist pin. These areas wear the most and cause the cylinder to become out-of-round because the piston is pressed up against them during compression and power stroke. During compression stroke the connecting rod is pushing the piston up on an angle. This along with the compression building above the piston, pushes the piston against the front of the cylinder. (inline engine) This area is called the minor thrust surface, it wears more because the piston is pressed up against it during compression stroke. During power stroke the connecting rod is also on an angle, but in the opposite direction. As the flame front pushes the piston down, the angle of the connecting rod forces the piston against the rear of the cylinder wall. (inline engine) The piston is pressed up against the cylinder wall much more violently than on compression stroke, that is why it is called the major thrust surface and is also why this area tends to wear the most.