But how does an engine work, exactly? Specifically, an internal-combustion engine is a heat engine in that it converts energy from the heat of burning gasoline into mechanical work, or torque. That torque is applied to the wheels to make the car move. And unless you are driving an ancient two-stroke Saab which sounds like an old chain saw and belches oily smoke out its exhaust , your engine works on the same basic principles whether you're wheeling a Ford or a Ferrari.
Engines have pistons that move up and down inside metal tubes called cylinders. Imagine riding a bicycle: Your legs move up and down to turn the pedals. Pistons are connected via rods they're like your shins to a crankshaft, and they move up and down to spin the engine's crankshaft, the same way your legs spin the bike's—which in turn powers the bike's drive wheel or car's drive wheels.
Depending on the vehicle, there are typically between two and 12 cylinders in its engine, with a piston moving up and down in each. What powers those pistons up and down are thousands of tiny controlled explosions occurring each minute, created by mixing fuel with oxygen and igniting the mixture. Each time the fuel ignites is called the combustion, or power, stroke. The heat and expanding gases from this miniexplosion push the piston down in the cylinder. Almost all of today's internal-combustion engines to keep it simple, we'll focus on gasoline powerplants here are of the four-stroke variety.
Beyond the combustion stroke, which pushes the piston down from the top of the cylinder, there are three other strokes: intake, compression, and exhaust. Engines need air namely oxygen to burn fuel. During the intake stroke, valves open to allow the piston to act like a syringe as it moves downward, drawing in ambient air through the engine's intake system. When the piston reaches the bottom of its stroke, the intake valves close, effectively sealing the cylinder for the compression stroke, which is in the opposite direction as the intake stroke.
The upward movement of the piston compresses the intake charge. As you can imagine, a piston has to be extremely durable to withstand all that energy. Still, problems can occur that can be minor and annoying or major and catastrophic. When the piston is worn and can rock side to side instead of moving vertically, the bottom edge of the piston, or skirt, contacts the cylinder wall.
Either from a defect, abuse, or lack of lubrication, a connecting rod can break away from the piston or crankshaft. It usually ends up taking out the rest of the engine as well. A rattle from the engine could mean the gudgeon pin, or wrist pin, has excessive play where it attaches to the piston. This wrist pin carries the full force of combustion.
The piston is not only subject to vertical forces during combustion, but also side forces caused by the continuously changing angle of the connecting rod. Because of these side forces, the piston needs smooth surfaces to run against the cylinder wall and keep the piston guided vertically upright. The side surfaces of a piston are known as the piston skirt.
There are two types of skirt. The most basic is a full skirt or solid skirt, which is the classic tubular shaped piston. This design is still used on truck and large commercial engines, but has long been replaced on cars and motorcycles by a lighter design known as a slipper piston. The slipper piston has part of the skirt cut away, leaving only the surfaces that bear on the front and back of the cylinder wall. This removal minimizes weight and reduces the area of contact between the piston and cylinder wall, thus reducing friction.
Modern production engines further reduce the friction between the piston and cylinder wall by using low-friction piston coatings , similar to teflon in a non-stick frying pan. These coatings are typically screen-printed on in a patch to the piston skirts - such as the illustrated graphite-based coating on a Ford Fiesta Ecoboost engine. As the piston is pushed down on the combustion stroke, it will exert a sideways force in the opposite direction to the angled connecting rod.
The direction of the cylinder on which this force acts is known as the thrust side, and both the piston and cylinder wall will suffer greater wear in this area. The piston gets incredibly hot, and needs to dissipate this heat efficiently.
The heat from a piston goes to three places: As radiant heat into the combustion chamber, into the cylinder walls via the piston rings and down the connecting rod. Additionally, many engines cool the piston through the use of oil sprayed onto the underside. The piston rings fit around the piston, bridging the small clearance between the piston and cylinder wall.
There are typically three piston rings on a piston and they perform different functions. The top two rings are called compression rings also known as pressure rings or gas rings and their main role is to prevent gases from getting through the small clearance between the piston and cylinder wall. This passage of gas past the piston and into the crankcase is known as blowby and should be minimized to maintain compression. Compression rings are typically made from solid cast iron and exert an outward pressure on the cylinder wall.
This outward pressure comes from the natural springiness of the rings, but is supplemented on the combustion stroke by gas pressure behind the rings which pushes them more tightly against the cylinder wall. The groove in the piston will be deeper than the width of the piston ring, allowing the ring to run on a film of oil. The compression rings also act to transfer heat from the piston to the cylinder wall, where it is dissipated into the coolant flowing through water jackets.
These rings are broken with a small gap which allows them to be installed and removed over the piston. The width of this piston ring gap will be specified by the manufacturer, and can be measured by placing the ring inside the cylinder and measuring the gap with a feeler gauge. The gaps are greatly exaggerated in this illustration, in reality they will be very thin - 0. The lower ring on a piston is an oil control ring. Oil is constantly sprayed onto the cylinder walls either from holes in the connecting rods or by jets installed in the crankcase.
For minimal friction, we need an thin oil film and the function of the oil control ring is to remove excess oil and leave an ideal oil film for the compression rings and piston skirt to glide over. The oil control ring will typically be made of two thin chrome scraper rings with a spacer sandwiched between them to allow oil be removed.
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