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How Car Engines Work - Not Your Basic Four-Stroke Guide

Everything You Ever Wanted To Know About The Process, But Were Afraid To Ask.

Mike Kojima
Mar 1, 2008
Photographer: HT Staff
Htup_0803_02_z+h22_vtec_honda_prelude_engine Photo 1/17   |   How Car Engines Work - Not Your Basic Four-Stroke Guide

At one time or another all of us have felt the hurt of misapplied modifications. Even the most respected names in the tuning industry have been pwned at least once. Those who have moved on to make power history surely had a short and direct learning curve, but some of us never seem to learn and just struggle to make power.

Are you tired of losing in front of crowds? Does your local tuning shop guru intimidate you with quasi-scientific BS? How do you know if that ROM tune or latest Chinese knock-off tempting you on eBay is any good? What exactly is a bigger camshaft? Are you bummed out by bad surprises on dyno days or getting pwned on those forums?

Htup_0803_03_z+honda_h22_engine_timing_belt_and_cam_gears Photo 2/17   |   Manipulating the four-stroke cycle is essential when looking for more horsepower so it's important to understand the different parts completely and how they affect overall power output.

If you fall into any of these categories, keep reading. The information is simple but it's essential for you to understand if your goal is to operate in the jargon-filled world of engine modding. Plus, if you want to go faster, you've got to understand how engines work.

Cars are, with the exception of rotaries, powered by the Otto four-stroke engine cycle. The name stems from the power cycle's four strokes: the intake stroke, compression stroke, power stroke and exhaust stroke. The cycle explains how an explosion of gasoline and air can be smoothly transferred into useable power, hurling you down that quarter-mile or just taking you to work.

Engine parts work in harmony in an exacting manner to harness gasoline's chemical energy converting small explosions of air and fuel into rotary motion. Honda aficionados can consider themselves fortunate. The company has some of the most advanced engines available for automobiles. Just by owning a Honda, you're ahead of the game.


The Block
The block contains the reciprocating components that harness gasoline's explosive properties. Pistons slide up and down cylindrical-shaped bores and the number of bores equals the number of cylinders. The block also contains cooling and lubrication passages for water and oil. Inside are main bearing saddles that support the crankshaft. Blocks are typically made of cast iron but Hondas are made of lightweight aluminum. Four-cylinder engines power most Hondas, although the Accord, NSX and various SUV types offer six-cylinder powerplants.

Htup_0803_07_z+honda_h22_pistons_and_rings Photo 3/17   |   Pistons are basically aluminum cylinders that slide up and down the block's bores. To work effectively they require thin, circular-shaped rings that seal off the chamber and prevent combustion pressure from blowing past the piston, losing power-producing pressure.

The Pistons
Simply put, pistons are cylinders of aluminum that slide up and down the block's bores, with the top of the bores being blocked off by the cylinder head. To make driving power, a flammable charge of compressed gasoline and air is ignited within the bore forcing the piston downward toward the cylinder's open end away from the cylinder head. This is the basic premise of how an engine works.

Pistons also have rings, which are thin, circular, springy metal seals that fit inside grooves around their tops. Rings help prevent combustion pressure from blowing past the piston and losing power-producing pressure. Rings also help scrape lubricating oil off cylinder walls to prevent oil from burning inside the cylinder. Without rings, it'd be impossible to develop enough compression to run, as well as burn up all that oil in just a few minutes of operation.

Htup_0803_06_z+honda_h22_connecting_rod Photo 4/17   |   Connecting rods transfer the explosion's force required to shove the pistons down their bores toward the crankshaft. The connecting rods look like metal dog bones connected to your pistons.

The Connecting Rods
Connecting rods transfer the explosion's force, shoving pistons down their bores toward the crankshaft. Connecting rods look like metal dog bones and are attached to the pistons by wrist pins-this would be the rod's small end. The rod's other end is attached to the crank. This is called the big end since the crank's journals are much bigger than the wrist pin's journals. Crank journals need to be bigger since the crank rotates at high speeds, unlike the wrist pin's simple rocking movement. This high-speed rotation requires additional bearing area to prevent the rod and crank from friction damage. The rod's big end spins smoothly on the crank's journal over a pressurized oil film and sleeve bearing-these are the rod bearings. On a typical Honda engine, the rod's small end has a bronze bushing for the wrist pin that's lubricated by oil splashing throughout the block.

The Crankshaft
Engine crankshafts are like bicycle crankshafts because they transfer up-and-down forces-the pistons being forced through the bore by the air/fuel explosion-into a rotating motion causing your wheels to spin. Cranks have offset throws, exactly like your bicycle's crank except the rods and pistons serve the same function as your legs do: they push the upward throw downward as the piston is pushed the same direction through the bore by the air/fuel explosion. This is what makes your car go. Once the piston goes down, the crank rotates and the piston is moved up again until it reaches the top where it can be pushed down once more by another air/fuel explosion. The crank rotates on its main journals on an oil-filmed sleeve bearing (the main bearings) just like the rods have for their big ends.

Htup_0803_08_z+honda_h22_16v_cylinder_head Photo 8/17   |   The cylinder head's combustion chambers contain the explosive forces needed for igniting the air/fuel mixture that ultimately drives the pistons downward.

The Cylinder Head
Honda cylinder heads are aluminum castings that cap off the tops of engine blocks. They house the spark plugs, combustion chambers, valves and valvetrain. The head must contain the explosive force for igniting the air/fuel mixture in order to drive the pistons downward and not escape. The combustion chambers are integrated within the cylinder head, which is where the valves and spark plugs are located. When looking at a cylinder head's underside (the side that mates to the block) the combustion chambers are the depressions that line up with the bores. It's inside these chambers, when the piston's at the top of its stroke, that the air/fuel mixture is ignited, kicking off the power stroke. The cylinder head also has cooling jackets filled with circulated water, which helps keep combustion chambers from overheating. The cylinder head contains the intake and exhaust ports, which are the passages where intake air and exhaust fumes pass when they're entering and exiting the cylinders.

Htup_0803_14_z+vtec_honda_timing_belt_and_cam_gears Photo 9/17   |   How Car Engines Work - Not Your Basic Four-Stroke Guide

The Valvetrain
Modern OHC (overhead camshaft) engine heads contain the intake and exhaust valves; both are spring-loaded poppet valves. The springs hold the valves shut but allow them to open with a push. The intake valves open to admit the explosive air/fuel mixture into the combustion chamber. They close to let the engine build compression as the piston, driven by the crank, reaches TDC (top dead center)-the point where the piston reaches the top of its stroke. When the spark plug ignites the mixture and the subsequent explosion drives the piston downward, the exhaust valves open near the bottom of the piston's stroke allowing burnt gasses to escape, preparing the combustion chamber for the next charge of fresh air and fuel.

Htup_0803_10_z+vtec_camshafts Photo 10/17   |   Valves open and close with the help of camshafts. Camshafts are basically rods with off-center bumps, or lobes, that spin inside the cylinder head at half the speed of the crankshaft.

Valves open and close via camshafts, which are basically rods with off-center bumps or lobes that spin inside the cylinder head at half the crankshaft's speed. The camshaft's lobes push the valves open and closed to admit both air and fuel and to expel exhaust. Some cam lobes work directly on the valves, like with many motorcycle and certain race engines. Typically, the camshaft works the valves through a rocker arm, which is like a miniature teeter-totter. One end of the rocker arm rubs on the rotating camshaft while the other end pushes the valves open and closed. The Honda engines you're familiar with use rocker arms.



Htup_0803_12_z+honda_h22_valvetrain Photo 11/17   |   Lucky Honda engines feature VTEC-Honda's highly effective variable cam timing system. The system affords the intake and exhaust cams two sets of lobes, one set optimized for low-rpm effectiveness, the other for high-rpm use. Honda's i-VTEC is similar but also allows for intake camshaft phasing adjustments on the fly for an ever-broader powerband.

Honda engines are overhead cam engines, which means the camshaft is contained within the cylinder head on top of its valves. This is different than OHV (overhead valve) engines like the low-revving, domestic V-8 with its block-located camshaft, connecting to its valves with lifters, pushrods and rocker arms. OHC engines are better suited for high-rpm, small displacement sport compact engines because they have simpler, lighter, more direct acting valvetrains. These valvetrains work better at higher engine speeds because their lower inertial mass allows them to follow the camshaft's lobes more accurately.

Htup_0803_11_z+sohc_honda_engine Photo 12/17   |   SOHC engines feature a single camshaft that controls all of the valves but many Honda engines feature DOHC configurations, which means there's a separate camshaft for the intake and the exhaust valves. The advantage here is that the camshafts can be placed closer to the valves, allowing its lobes to either work directly on the valves or through smaller rocker arms. This keeps the valvetrain's inertial mass to a minimum, which helps high-rpm operation even more.

SOHC (single overhead camshaft) engines have just one camshaft that controls both the intake and exhaust valves but many Honda powerplants feature dual overhead cams, meaning there's a separate camshaft for the intake and the exhaust valves. The advantage here is that the camshafts can be placed closer to the valves and allow its lobes to either work directly on the valves or through smaller rocker arms. This keeps the valvetrain's inertial mass to a minimum, which helps high-rpm operation even more. Most high-performance Honda engines use dual overhead cam valvetrains, also known as the DOHC configuration.

Select Honda engines have one of the greatest innovations for performance-minded, small displacement engines-VTEC. VTEC is Honda's unique and highly effective variable cam timing system. The system affords the intake and exhaust cams two sets of lobes: one set optimized for low-rpm effectiveness, the other for high-rpm use. This gives the engine a wider operating range, enabling one to build something that's quite docile at lower engine speeds yet remain capable of high-rpm screaming. Honda's i-VTEC is similar but also allows for intake camshaft phasing adjustments, advancing and retarding it to alter cam overlap for a broader powerband.

Htup_0803_09_z+intake_manifold Photo 13/17   |   The intake system consists of the intake manifold, featuring an open chamber, or plenum, attached to a series of pipes that span from the plenum to the cylinder head's intake ports. The throttle body serves as an air-metering valve and is mounted to the plenum's end.

The Intake System
The intake system consists of the intake manifold featuring an open chamber, or plenum, attached to a series of pipes that span from the plenum to the cylinder head's intake ports. The throttle body serves as an air-metering valve and is mounted to the plenum's end. The throttle body controls the amount of air the engine sucks in, thus controlling engine speed and horsepower. When it's shut, air is limited so the engine is forced to idle. When wide open, the engine ingests all it can to produce the maximum power it's capable of. The manifold contains the fuel injectors, which are electro-mechanical valves controlled by the ECU-the engine's brain. The ECU controls the amount of fuel injected by modulating the injectors' tiny valves' open and close time. During idle conditions, only a small amount of fuel is injected but with the throttle fully opened, allowing additional air to be ingested, the ECU signals the injectors to remain open longer to inject a proportionally greater amount of fuel. More fuel and more air equal bigger explosions and more power to the wheels.

The Ignition System
An electrical spark timed by the ECU and fired across the spark plug's electrodes ignites the flammable air/fuel mixture in the cylinders. The spark fires just before the piston reaches TDC, near the peak of the cylinder's highest compression pressure. This is the most efficient time to fire the spark. Usually, spark timing advances alongside engine speed because at higher engine speeds there's less time for combustion events to take place so it must be started sooner in the cycle to maintain proper operation.

Htup_0803_16_z+honda_d_series_engine_header Photo 14/17   |   How Car Engines Work - Not Your Basic Four-Stroke Guide

The Exhaust System
The tubing directing burnt exhaust gases away from the engine is the exhaust system. This includes the exhaust manifold, catalytic converter and exhaust piping. The manifold collects each of the cylinder head's exhaust port's waste gas and collects it into a single pipe. This leads into the catalytic converter where poisonous constituents of the exhaust gas, like nitrogen oxide, various unburned hydrocarbons and carbon monoxide, are converted into non-toxic carbon dioxide and water vapor. From there gases flow into the exhaust pipe where they pass through the muffler, reducing noise to an acceptable level and then out into the atmosphere.

The Intake Stroke
With a basic understanding of an engine's moving parts under your belt, now's a good time to explain the four-stroke process and find out how everything works together. Manipulating the cycle is essential when looking for more horsepower so it's important to completely understand the different parts and how they affect overall power output beginning with the intake stroke.

Htup_0803_13_z+honda_h22_cam_system Photo 15/17   |   How Car Engines Work - Not Your Basic Four-Stroke Guide

Let's start with the piston at TDC. The intake valve begins to open as the exhaust valve closes. As the crankshaft turns, the connecting rod starts pulling the piston downward, away from TDC. Keep in mind that the crank is linked to the camshaft by a chain or belt so, as the crank turns, the intake valve opens further until fully open. The downward traveling piston creates suction in the cylinder so air and injected gasoline from the intake manifold are drawn inside. This continues until the piston reaches BDC (bottom dead center). Because of the camshaft's shape, the intake valve is almost totally closed by the time the piston reaches BDC. By the intake stroke's end we're left with a cylinder full of a fresh air/fuel mixture.

The Compression Stroke
By now the piston is beginning its upward trip, being pushed by the crankshaft and connecting rod combo. The intake valve is fully closed and, as the piston's forced upward, the air/fuel mixture is compressed. This compression forces the air and fuel molecules closer together until they become a highly reactive explosive mixture. The closer the molecules' proximity to one another, the easier it is to initiate an explosion. As the piston nears TDC once again, the ignition system fires the spark plug that triggers another explosion inside the cylinder.

Htup_0803_01_z+four_cylinder_camshaft_and_valve_interval_graph Photo 16/17   |   The diagram above illustrates valve events in relation to crankshaft rotation. Notice that two crankshaft revolutions are required - a total of 720 degrees of rotation - for each full camshaft revolution, or 360 degrees of rotation.

The Power Stroke
By the time the piston is at TDC, the air/fuel explosion inside the tightly contained cylinder is well under way. The explosion's heat and pressure increases rapidly and the piston's pushed back down its cylinder with great force. This is the driving power that spins your wheels and propels you down the track. As the piston is pushed downward and cylinder volume increases, cylinder pressures decrease. Once the piston nears the bottom of the bore, the camshaft begins opening its exhaust valve.

The Exhaust Stroke
There is nothing too exciting going on here. As the piston once again travels from BDC upward, the exhaust valve opens and burnt gases are forced out of the cylinder into the exhaust system. By the time the piston reaches the top of its bore, the exhaust valve is just about closed and the intake valve is beginning to open and the cycle repeats.

Htup_0803_17_z+fully_built_b18c5_race_engine Photo 17/17   |   How Car Engines Work - Not Your Basic Four-Stroke Guide

Each cylinder of any given four-stroke engine experiences its four strokes for every two crankshaft revolutions. Since camshafts have one bump for each lobe and are driven at half the crankshaft's speed, the valves open with every other crankshaft revolution. Imagine this happening at even a conservative 7,000 rpm where the cycle would repeat itself at a rate of roughly 60 times per second per cylinder. At speeds like this it's easy to imagine a more uninterrupted flow of power coming from this seemingly herky-jerky system.

By Mike Kojima
55 Articles

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