How Engines Work

This is a typical four-cycle...        read full caption This is a typical four-cycle engine overhead valve (OHV) cylinder arrangement. Piston, cylinder, connecting rod, crankshaft, combustion chamber, spark plug, intake, and exhaust valves all work together to make power.

During intake stroke, the...        read full caption During intake stroke, the piston moves downward in the cylinder. With the open intake valve, the piston draws fuel and air into the cylinder. This is intake stroke or cycle 1. Think of this like you would gunpowder in a shell.

As the crankshaft comes around,...        read full caption As the crankshaft comes around, the piston begins its travel back up the cylinder bore. With both valves closed, this becomes compression stroke (cycle 2) where the piston squeezes the fuel/air mixture. When we squeeze the fuel/air mix, we get heat.

With our fuel/air mixture...        read full caption With our fuel/air mixture squeezed and hot, the spark plug fires, igniting the fuel/air combo. This is power stroke (cycle 3) where rapidly expanding hot gasses exert pressure on the piston, thrusting it downward in the cylinder, turning the crankshaft. Think of this like the hammer striking the shell and firing the bullet.

The hot, expanding gasses...        read full caption The hot, expanding gasses have done their job and given us power. As the crank revolves, the piston begins its journey toward the top of the cylinder again. This time, the exhaust valve opens, allowing the piston to expel the spent gasses. Next, the intake valve will open and the process begins again.

This is a head-on look at...        read full caption This is a head-on look at an inline six-cylinder engine. From this perspective, we see only one cylinder, but five are behind it. Note the distributor, which is tied to the camshaft, which is tied to the crankshaft via a timing chain and gear set. The oil pump, which moves oil through the engine from the pan, is also visible here.

This is a V-8 engine with...        read full caption This is a V-8 engine with two banks of four cylinders. Here, we see only two cylinders. Each has three cylinders behind it. A V-6 is the same principle, with three cylinders on each bank.

The crankshaft, connecting...        read full caption The crankshaft, connecting rod, and piston do the engine’s real work. The pressure of hot gasses acts on the top of the piston, which moves the piston and rod downward in the cylinder bore, which turns the crankshaft. This is a Ford “FE” series big-block cutaway view, which allows us to see the engine’s insides.

Piston rings provide the seal...        read full caption Piston rings provide the seal between piston and cylinder wall. The top two compression rings give us the seal. The oil control rings carry lubrication up and down the cylinder wall without allowing it into the combustion chamber.

Sitting on a work bench, an...        read full caption Sitting on a work bench, an engine’s valves don’t look like much, but they have the toughest job in your engine. They act as traffic cops for extremes—the cold dampness of intake stroke and the hellish hot of spent exhaust gasses. Plus, they hold in the great pressures of compression stroke.

This is a typical small V-8...        read full caption This is a typical small V-8 Ford combustion chamber. Here, the intake valve is open, which would allow the fuel/air mix into the chamber above the piston. Note the spark plug firing tip amid the valves. The intake valve is always larger than the exhaust valve.

The exhaust valve is open...        read full caption The exhaust valve is open to allow used gasses to escape.

This is a typical camshaft....        read full caption This is a typical camshaft. As you can see, the lobes or eccentrics are there to open intake and exhaust valves in a timed sequence.

Here, we see the valvetrain,...        read full caption Here, we see the valvetrain, which gets the valve open. Lifters ride the rotating camlobes. As lifters ride the camlobes, they transfer motion to the pushrod, rocker arm, and valve.

Here’s one example of...        read full caption Here’s one example of what we might see out there. This is a 351 Windsor V-8 cylinder head with its valvetrain partially installed. Note the rocker arm, pushrod, valve, and spring.

Talk about wild and crazy?...        read full caption Talk about wild and crazy? This is a BOSS 429’s rocker-arm assembly. Because the BOSS 429 has huge cylinder heads with hemispherical chambers, it needs long rocker arms like these. These long rocker arms open the exhaust valves. Shorter rocker arms open the intake valves.

This is a 351 Cleveland cylinder...        read full caption This is a 351 Cleveland cylinder head with canted valves. Canted means the valves are at various angles. Ford calls them “poly angle” valves. These are roller rocker arms for high-performance use.

The crankshaft turns the camshaft...        read full caption The crankshaft turns the camshaft with a timing chain and gearset. Timing is very critical here.

To get the timing perfect,...        read full caption To get the timing perfect, we must align timing marks on the cam and crankshaft gears as shown. The fuel pump eccentric works the fuel pump.

The old Autolite dual-advance...        read full caption The old Autolite dual-advance distributor, as its name implies, has two types of advance mechanisms. Vacuum advance gets its motivation from manifold vacuum based on throttle position and load. The centrifugal advance functions based on engine speed. The faster the engine revs, the more the centrifugal flyweights advance the rotor.

This is a typical point-triggered...        read full caption This is a typical point-triggered ignition system. When we turn the ignition switch to “on,” we route 12 volts of electricity to the ignition coil. As ignition points inside the distributor open and close in time with the engine crankshaft and camshaft, the ignition coil builds the high voltage necessary to fire spark plugs.

This is an Autolite dual-point,...        read full caption This is an Autolite dual-point, high-performance distributor, which does not have a vacuum advance unit. Dual-point ignition with a centrifugal advance does its work at high rpm.

This very simple illustration...        read full caption This very simple illustration shows how coolant flows through the engine and radiator. Coolant flow is controlled by the thermostat, which regulates flow from the engine to the radiator.

When engine coolant gets hot,...        read full caption When engine coolant gets hot, the thermostat opens, releasing hot coolant into the radiator. Lower temperature coolant flows from the radiator into the engine. The cooling system’s job is simple. It pulls heat out of the engine.

This is a typical engine oiling...        read full caption This is a typical engine oiling system. Oil is drawn from the pan by the pump, which feeds oil under pressure through the filter, then onward to the engine’s oil galleys. Crankshaft and camshaft bearings receive oil first, then lifters, rocker arms, and valves.

When we turn the key, what happens under the hood and why? How does an automobile engine work? What is that sound we hear and the propulsion we feel when the accelerator is depressed?

We know gasoline is flammable. It burns. And we know gasoline creates heat energy when it ignites. Heat created by the igniting gasoline and air mixture is what gives us the power necessary to propel our Mustangs. With all of this in mind, let’s talk about internal combustion engines. “Internal combustion” means we burn fuel and air inside the engine to create heat and generate the power necessary to propel a car.

“External combustion,” for example, is similar to that found with a steam engine. Fuel is burnt outside of the engine in a boiler to heat water that becomes steam and heat energy. With both internal and external combustion engines, heat energy (expansion) exerts force on a piston and connecting rod tied to a crankshaft to make rotary motion. In physics class, we call this turning linear motion into rotary motion.

Okay, so what does all of this “linear” and “rotary” stuff mean? Think about what you see when you’re watching an old episode of Petticoat Junction. The Cannonball steam locomotive to nearby Pixley has a series of arms tied to pistons and wheels. Steam pressure moves the piston, which moves the arm, which turns the wheels to propel the locomotive. Pistons and arms make linear (back and forth) motion. Wheels make rotary (around and around) motion. The wheels are counterweighted to help momentum.

“Chuga-chuga-chuga-chuga” motion that comes from steam pressure gets us down the track. Your Mustang’s engine works on the same principle, only the “chuga-chuga-chuga” motion is inside the engine, invisible from the outside.

The crankshaft, like locomotive wheels, is counterweighted for balance and momentum. Around and around it goes—channeling energy to your Mustang’s transmission, driveshaft, and rear axle.

Burn, Turn and Roll
Mustangs are equipped with four-cycle gasoline engines. The four cycles are intake, compression, power, and exhaust. How do we make power from the four cycles? Air and fuel must first be mixed into a vapor or mist before entering the combustion chamber above the piston. Liquid gasoline, as a rule, doesn’t burn. If you were to throw a lighted match into a bucket of gasoline (do not do this!), the liquid wouldn’t burn; the fumes above the fuel ignite and burn. When gasoline becomes vaporized (mixed with air), it ignites with fury, which makes heat and pressure to do our work.

Fuel becomes a vapor two basic ways. Carburetors, common in Mustangs prior to 1986, mix fuel and air to create the vapor needed to support combustion. Electronic fuel injection atomizes (vaporizes) fuel under pressure either at a single entry point (central fuel injection) or at each intake port (sequential electronic fuel injection). Central and sequential fuel injection are both computer-controlled systems.

Vaporized fuel is drawn into the combustion chamber by the moving piston in the cylinder bore. An open intake valve provides the entry point. This is called intake stroke. When the piston reaches the bottom of the bore, the intake valve closes, terminating the entry of fuel and air. When the piston begins its journey back to the top of the cylinder bore, it squeezes the fuel and air against the closed valves. This is called compression stroke. As the piston nears the top of the cylinder bore, the spark plug fires, igniting the fuel/air mixture. The heat and pressure created during ignition exerts force on the piston, pushing it downward in the cylinder bore, applying pressure on the connecting rod and crankshaft. We call this the power stroke. This linear (straight line) force turns the crankshaft, becoming rotary motion. As the piston nears the bottom of the cylinder in the power stroke, the exhaust valve opens. The piston begins its journey back to the top of the bore, forcing exhaust gasses out through the open exhaust valve. This is called the exhaust stroke. Our engine’s four power cycles are complete.

We have introduced you to the workings of a single cylinder. If Mustangs had only one cylinder, there wouldn’t be enough power to get the job done. Since 1964, Mustangs have been getting the job done with four, six, and eight cylinders. From 1964-’73, Mustangs came standard with six cylinder engines—with six cylinders positioned in a row along a long crankshaft.

Eight-cylinder engines have always been optional, with eight cylinders in a “V” configuration on two banks of four cylinders each. Beginning in 1974, standard Mustang power was four cylinders in a row. When we line up four and six cylinders in a row, we call it an “inline” engine. Economy cars are traditionally equipped with inline fours and sixes. These engines make a buzzy sound. V-type eights, or V-8s, make a throaty sound much different than inline engines. If this doesn’t make sense to you, think of it this way. NASCAR Winston Cup racing consists of V-8 engines that make that powerful roar. Busch Series and Grand National racecars get power from V-6 engines that buzz like a swarm of bumblebees. From 1974-’79, 1982-’86, and 1994-’03, six-cylinder Mustang power isn’t inline, but instead a V-6, with two banks of three cylinders in a “V” formation.

Stop Cocks and Bumpsticks
How do we get the fuel/air mixture and exhaust gasses into and out of the combustion chamber? We do this with poppet valves (stop cocks). One valve allows the fuel/air mixture in and another lets hot exhaust gasses out. Poppet valves are shaped like large nails with huge heads. The tapered valve heads close against a tapered seat in the cylinder head to stop the flow. The closed valves seal the combustion chamber between intake and exhaust cycles.

How do poppet valves work? Poppet valves are held closed by springs known as valvesprings. The camshaft (bumpstick), a rotating shaft with a series of eccentrics or lobes, opens the valves in time with the crankshaft. The crankshaft and camshaft are tied together with a timing chain and gearset at the front of the engine. Camshaft speed is normally half that of the crankshaft. This makes perfect sense when we consider the four cycles that give us power. The crankshaft makes two complete revolutions for every revolution of the camshaft. Another way to look at this is we have two complete revolutions of the crankshaft for the four cycles. When the crankshaft is whirling around at 4,000 rpm, the camshaft is spinning at 2,000 rpm.

How do cam lobes open valves several inches away? They do this via lifters, pushrods, and rocker arms. The lifter rides on the cam lobe. The pushrod sits in the lifter and transfers linear (back and forth) motion to the rocker arm at the cylinder head. The rocker arm is a lever that takes the pushrod’s linear motion and transfers it to the poppet valve. The poppet valve is opened by the cam lobe and closed by the valvespring.

Light My Fire
To have power at all, we need a way to ignite the fuel/air mixture once it is inside the combustion chamber. We do this with a timed, high-voltage spark. The timed spark comes from the ignition system, which is tied to camshaft and valve timing. Mustangs prior to 1996 have a distributor, which times the spark at compression stroke as the piston nears the top of the cylinder. The distributor channels high-voltage electricity from the ignition coil (an electrical transformer) to each of the engine’s spark plugs. Each cylinder (combustion chamber) has one spark plug. If we have a six-cylinder engine, we have six spark plugs. A V-8 has eight.

When we think of a multi-cylinder engine as four, six or eight individual engines, it becomes easier to understand spark timing. In a V-8 engine, for example, we have eight individual cylinders firing at eight different times in sequence. No two cylinders fire at the same time. They fire in a sequence known as the firing order. Your Mustang’s 289ci V-8 engine, for example, has a firing order of 1-5-4-2-6-3-7-8. If you study this firing order, cylinders fire back and forth across the two banks. Cylinder 1 fires first. Then cylinder 5 on the opposite bank. Then back across to cylinder 4. Then across the same bank to cylinder 2. Then over to the opposite bank to cylinder 6. Then back over to cylinder 3. And finally cylinders 7 and 8.

With eight separate engines working together on a common crankshaft, we have a continuous distribution of power from eight cylinder bores. The distributor makes it possible to fire these eight cylinders in proper order. When we tie the distributor to the camshaft and crankshaft, it all works together in near-perfect synch.

Understanding Old Sparky
We know the ignition system has to fire multiple spark plugs in a timed sequence. Each spark plug has a small gap between the center electrode and cross electrode. Vintage Fords typically have a spark plug electrode gap of .034-inch, suitable for low-energy, point-triggered ignition systems. To get a spark to jump any sized gap, we need high-voltage electricity powerful enough to jump the gap. We produce high-voltage electricity via the ignition coil, which is an electrical transformer that transforms battery voltage (12-14 volts) to high-voltage electricity (around 20,000 volts).

Inside conventional point-type distributors, the ignition points act as a switch for the ignition coil’s task of producing high-voltage electricity. When the points open, the ignition coil releases the store of electricity to the distributor’s high-tension lead. Because this is a timed sequence, the distributor rotor will be positioned on a given cylinder’s spark plug wire lead when the points open. With a V-8 engine, this happens eight times in two revolutions of the crankshaft. With electronic ignition, a magnetic pickup module inside the distributor takes the place of ignition points and condenser. This is oversimplified, of course, because there is also an ignition amplifier separate from the distributor that works hand-in-hand with the pickup module.

The four-cycle power process would be easy to handle if all we had to worry about was firing a spark plug when the piston reached the top of the cylinder. But if we fired the spark plug when the piston reached top-dead-center (TDC), we wouldn’t make the most of the fuel/air mixture. Because fuel takes a given amount of time to ignite and burn, we have to fire the spark plug before the piston reaches the top of the cylinder. Fuel and air don’t “explode” in the combustion chamber. The fuel/air mix ignites in a “quick fire” that begins at the spark plug and roars across the top of the piston. This is called a flame front.

As engine speed increases, the spark has to occur earlier in the compression stroke. Because we’re locating the spark earlier in the compression stroke, this is called “advancing” the spark. How do we advance the spark? We do this with two functions inside the distributor—vacuum advance and centrifugal advance. Centrifugal advance works with the speed of the engine. Vacuum advance works when we lean on the throttle. Both work together to adjust spark timing in line with engine power demands.

Centrifugal advance works with engine speed via springs and flyweights that position the distributor rotor. The faster the distributor turns, the further out the flyweights move against spring tension, positioning the rotor earlier in the compression stroke. Spark timing happens in degrees of crankshaft rotation and piston location. Crankshafts rotate 360 degrees in a full circle. As the piston makes its way up the cylinder bore on compression stroke, this happens between zero and 40 degrees of crank rotation before the piston reaches the top of the bore.

At idle speed, we want the spark to occur at 6 to 12 degrees (crank rotation) before the piston reaches the top of the cylinder. This is called BTDC (Before Top Dead Center). When the engine revs to 3,500 rpm and higher, the spark needs to happen earlier around 36 to 40 degrees BTDC. This helps our engine make the most of the fuel/air charge. When we move the spark any earlier than 40 degrees BTDC—and this is pushing our luck—we risk premature combustion (pinging and detonation) that can do serious engine damage.

Cooling It
All that heat energy released in each combustion chamber during power stroke can be destructive if it isn’t controlled. We control the heat with water jackets around the cylinders and in the cylinder heads. Water jackets contain the coolant mix that keeps our engine temperature stable. This is done with coolant, the radiator, a thermostat, hoses, water pump, and a fan that pulls outside air through the radiator.

Coolant flows into the engine’s water jackets via the bottom radiator hose. Ideally, we have water jackets that are completely full of coolant, without air bubbles that cause hot spots. The thermostat is a flow-control valve that opens when the coolant temperature in the engine reaches either 180 or 192 degrees Fahrenheit. Older Mustangs have a 180-degree thermostat. Newer, computer-controlled Mustangs are 192 degrees.

When the thermostat opens, it allows hot coolant to leave the engine and flow into the radiator through the upper hose. Coolant in the radiator is displaced by the incoming hot coolant from the engine. Fresh coolant in the radiator enters the engine, which closes the thermostat. The cycle begins all over again when coolant inside the engine reaches 180 or 192 degrees.

Lube It Or Lose It
Another engine survival tool is the oiling system. Engine oil is stored in the oil pan at the bottom of the engine. When we check oil, we’re pulling a dipstick that shows us how much oil is in the pan. The camshaft we mentioned earlier also has the busy task of turning the oil pump that keeps our engine alive. It does this through a shaft splined into the distributor shaft.

Our Mustangs have what is called a “G” rotor oil pump that draws oil from the pan and forces it into the oil galleys that feed moving parts like crankshafts and camshaft bearings, pistons and cylinder walls, lifters, rocker arms, timing gearset, and more. Oil is fed to these critical moving parts under pressure, which keeps them from rubbing together. Moving parts float on a layer of engine oil. Think of engine oil as a barrier that protects moving parts from certain destruction. Another thing engine oil does is cool hot engine parts. As oil flows through the engine, it carries heat away from the hottest parts, such as bearings, pistons, rings, and even valve stems. Engines fail whenever oil pressure is lost and moving parts grind themselves to an unpleasant halt.