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RPM to MPH: How Car Engines Work

Text by Dmitriy Orlov


Getting Started

The Building Blocks Series is designed to give a comprehensive view of how vehicles work, "zooming in” on details to really see the inner workings of all the components but at the same time "zooming out” to see the bigger picture and how all the details come together. The series will cover the major components and concepts of a car and will also dive in, further dissecting the mechanics.

The first article of the series focuses on the "heart” of a car—the internal combustion engine, which moves the car, gives it power and acceleration, and emits that noise that so many enthusiasts love to hear.

The internal combustion engine as we know it today has been around since the late 1800s. Over the years, engineering and material developments have greatly improved on designs and performance aspects of the engine, but the theory and physics behind its operation remain more or less the same.

In this article, we are concerned with the major parts of an engine: the block, heads, rotating assembly, how these work together, and the internal dynamics of what makes the engine turn over and create power.




Cause and Effect

The way an engine works is rather simple—converting energy stored in a fossil fuel into mechanical energy. The fossil fuel used in an engine is most commonly the regular fuel you get at a gas station, be it gasoline or diesel. This fuel is injected and mixed, compressed and ignited, creating pressure to push down on a piston, which then helps rotate a whole assembly with other pistons. This rotation is what ultimately propels a car.

Tweaking all the variables in the engine varies its output, response, and strength. Modern technology and materials allow engineers to optimize the use of fuels and reduce the weight and size of engines to a point where smaller engines are now as powerful if not more so than older, much larger engines.

The next sections explore variants of internal combustion motors, the motions in an engine that harness the energy of fuels and the components that make it all happen.

Variants

There are many engine classifications and details—all are better suited for various applications—whether for a car or a truck, a generator, or even an airplane.

A 4-stroke motor, also known as the Otty Cycle engine, is the most common across all vehicles. It has a wide range of applications and has been perfected to be both very efficient in fuel consumption and power output.

Diesel engines are a little different from Otto Cycle motors. While using the same basic priciples, they run on diesel and under much higher pressures and utilize compression ignition rather than the spark ignition of the Otto Cycle engine. The diesel engine is considered the workhorse of all engines—commonly found in trucks, tractors, and anything else that requires moving weight. It is not quite suited for speed, but the torque output of these engines can pull around most things on the planet.

Creative

Variations of internal combustion motors have also been designed, such as the rotary engine known as the Wankel. It has a simple and beautiful design, yet has been quite inefficient and requires high maintenance in order to be used successfully in mass-produced cars.

Cycles

The Otto Cycle motor operates on the basic principle of generating a contained explosion that creates enough force to move a piston to rotate a crank, which, in turn, is the source of motion for the vehicle. Four strokes are involved in operating the motor, hence the 4-stroke name.



These four cycles refer to intake, compression, combustion, and exhaust cycles that occur during two crankshaft rotations per power cycle of the engine. The cycle begins at Top Dead Center (TDC), when the piston is farthest away from the axis of the crankshaft. A cycle refers to the full travel of the piston from Top Dead Center (TDC) to Bottom Dead Center (BDC).

On the intake or induction stroke of the piston, the piston descends from the top to the bottom of the cylinder, reducing pressure inside the cylinder. A mixture of fuel and air enters the cylinder through the intake port. The air is sucked in if the motor is naturally aspirated or can be forced in through "forced induction” systems, such as turbochargers or superchargers. The intake valves then close. The volume of air/fuel mixture that is drawn into the cylinder relative to the volume of the cylinder is called the volumetric efficiency of the engine. The camshafts that control the opening and closing of the valves are part of the valve train and largely determine the volume of mixture. The whole assembly has to be perfectly timed with crank rotation and piston position.

With both intake and exhaust valves closed, the piston returns to the top of the cylinder, compressing the air or fuel-air mixture into the combustion chamber of the cylinder head.

The power stroke starts the second revolution of the engine. While the piston is close to Top Dead Center, the compressed fuel-air mixture is ignited, usually by a spark plug. The resulting massive pressure from the combustion of the compressed fuel-air mixture forces the piston back down toward bottom dead center. The piston itself sits on a connecting rod, linking it to the crank. The connecting rod is key in translating lateral movement into rotation.
During the exhaust stroke, the piston once again returns to top dead center while the exhaust valve is open. This action evacuates the burned products of combustion from the cylinder by expelling the spent fuel-air mixture out through the exhaust valves.

The whole operation was initially a mechanical "cause and effect” setup, where the initial rotation of the crank was made by way of a starter, which then made everything else come alive after the first few ignitions.

Modern engines are controlled by advanced computer systems that monitor every movement of the engine to ensure it runs properly and strong. Computers help with emissions control as well as optimizing power output, manipulating different variables around the engine.

Support System


With all the cycles in motion in the motor, it is crucial that all components are free to rotate, slide, and operate. An oil pump, running off the crank, ensures that oil is pumped throughout the entire block and valve train to ensure smooth operation by adequately lubricating the components, which is why running low on oil can damage and even lock up a motor.

The operation of the motor also generates considerable heat—from explosions in the cylinders or the friction produced from all the components. A water pump ensures a steady flow of coolant through passages in the block and heads, which take heat away and dissipate it in the radiator of the vehicle. The same coolant is used by the heater core inside the cabin to provide heat for passengers in cold weather.

Configurations

Various engine configurations have been designed to accommodate more cylinders (for more power output) or to better fit space constraints. Most common are the inline engines—where the cylinders are all lined up straight—these range from four to even eight cylinders for a single engine. Another setup is a the V-type setup, where cylinders are angled at varying degrees to each other—commonly called V6, V8, V10, V12, and more. The advantage is having more cylinders in a more compact size. A flat engine means the cylinders are set opposite each other on either side of the crank. This arrangement provides for an engine with a very low profile, which is great for lowering the center of gravity for the vehicle.


Flat 4-cylinder engine aka "Boxer" engine



Inline 4-cylinder engine



"Vee" 8-cylinder engine aka V8 engine

RPM to MPH

Your engine functions through a series of controlled explosions, propelling pistons to rotate a crank by pushing down on rods. You also have valves opening and closing and sparks igniting, all at just the right split-second moment—all controlled by a microprocessor. All you have to do is turn the key or a push button.

But how do you start moving? The engine is spinning, and you shift into gear (with or without a clutch, depending on what you are driving).

The lateral motion of the pistons travelling up and down on the rods rotates the crankshaft, which is connected to the transmission—the unit that converts the engine’s power into forward (or reverse) motion.

The next article in the series will discuss how power from the engine is transferred to the wheels through the transmission and drivetrain.

Previous Articles
An Introduction to the Building Blocks Series


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JonHerrman5/21/2014

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