Internal Combustion Basics: Converting Heat To Pressure
If you have a go kart, a weed wacker or a lawn mower for that matter you probably have wondered how it actually works. The modern internal combustion engine is really a marvel when you sit down and think about the things that it has accomplished over the past 100 years.
Actually development of the burning of fuel in a cylinder was a result of the steam revolution, where instead of providing heat to the water a more direct approach was being considered.
Engines are heat converters as a whole. Even a nuclear power plant is nothing more than a conversion of heat to useable energy. We often get starry eyed when we watch Star Wars or Star Trek or any Sci-Fi movie because we see all these wondrous mechanisms that do impossible things like float, go the speed of light and so forth. The flux capacitor for example…
Some of your more credible movies will make an attempt at explaining their power systems and invariably what they end up doing is tapping the Energy from the Atom and using it directly. For the most part science has not gotten that far yet.
The most direct energy transfer that we can attain is through magnetic fields. Magnetic fields are an area left for the most part untapped. There is a great mystery about its relationship to atoms and their motion, to gravity and light. But as a whole, we do not understand it, and we have just skimmed the surface in it practical applications.
In general we use magnetic fields to generate electricity, we use magnetic fields to contain plasma in a nuclear experiments and reactions, we use magnetic field to run motors, we use magnetic fields to polarize cells for MRI machines.
But alas, we are not intending to talk about Magnetic fields but internal combustion engines. Internal combustion engine is a label given to describe an engine that burns fuel internally and converts the heat energy into rotational motion.
The gas law which is described as:
PV = MRT
P = Pressure
V = Volume
M = Mass
R = Gas Constant
T = Temperature
The above formula is designed to create a relationship between the four variables: pressure, volume, mass and temperature. (R is a constant that is plugged into the formula to make it useable.)
If we step back and rewrite the formula simplistically so we can evaluate two of the most commonly used variables it will become apparent how the relationship works:
P = T *K
(K is a constant that combines all the other elements, such as volume and mass and the basic constant. In this example Volume is constant, and we are not changing the mass.)
The variable I want to play with is the Temperature. If I increase the temperature, what has to happen to the pressure?
P+++ = T+++ *K
To make the equation balance, the P must increase as well. P is pressure, you know the stuff that makes a balloon big. When the balloon pops we hear a sudden pressure increase, called a bang. All sound waves are pressure increases or minute temperature rises as well.
Another way to look at pressure is that it is a force that covers a big area. If you take one square inch put 1 pound of force on that square inch you have 1 pound over that square inch.
If you take a door which is 40” by 80” and put 1 lb/square inch on it you will have 3200 lbs of force on the door. In the same way force is applied to the face of a piston, or on the face of a propeller or turbine blade.
Correspondingly, if you every watch popcorn in the microwave it expands significantly because of the heat that is escaping from the popping cornels. In this case steam is expanding the bag.
In the case of water, it changed from water to a gas phase or steam. The steam expands like it should when heat is applied. In fact, in modern power stations the steam is superheated, meaning it is increased beyond 212 degrees F to much higher on the order of 400+ degrees. The increasing of the temperature of the steam increases its pressure correspondingly.
The real useful element in all engines is not the temperature so much as it is the pressure. Pressure is the element used to propel blades, pistons, bullets, grenade fragments, knock over buildings…. Pressure is what causes a piston to move down the cylinder, pressure is what causes a wind turbine to have its blade move, pressure is what keeps an airplane flying, pressure is the practical power mover in all heat engines.
For all intensive purposes, the major link between pressure and temperature is the big struggle for power and energy conservation. Increasing the efficiency of the link between pressure and temperature is the biggest struggle, because heat can readily escape and not be converted into pressure. It escapes into the metals of the engine.
Increasing the intensity of the switch over from Heat to pressure is the big struggle.
If you study the history of heat engines you will see a drive towards increasing the usefuleness of the heat to pressure relationship. Additionally, the utilitarian aspect of the engine drove it immensely. In other words an engine that was huge but put out only 3 horsepower would not suite being put into a go kart. The drive is to have an engine that puts out power, but is light.
So the internal combustion engine combines compactness, and immediate heat transfer to pressure. Instead of burning a gas outside of the engine under a boiler, converting the heat to steam pressure and then piping the steam pressure to the piston, the fuel is burned directly in the cylinder, causes an immediate temperature increase, and instantly causes a pressure rise which forces the piston down the cylinder.
All internal combustion engines use this process: burn the fuel in the cylinder to cause an immediate pressure rise. For example a jet engine has a chamber for burning fuel. The burning fuel causes an immediate temperature rise and a corresponding pressure rise. This pressure rise pushes against the turbine blades, or shoves them out of the way. The turbine then pushes on the shaft cause it to rotate, much like a glorified propeller, or pin wheel.
A rocket engine is actually a very simple conversion of heat to pressure. The combustible fuel combination is dumped into a chamber and exits out one side of the chamber. The pressure in essence is directed like a fire hose and comes out as a stream of fire. An erratic let loose balloon is the same concept. The pressure of the escaping air cause a force that propels the balloon around the room.
In a go kart engine the gasoline is being converted from burning fuel to useful rotational energy. The burning fuel expands forcing the piston down the cylinder. The rest of the mechanics in the engine is designed to harness that pressure and to enable the engine to have more than one piston firing.
There are two types of engine cycles predominately used in the world to day and they are:
1. Four Cycle
2. Two Cycle
In the historical development, the four-cycle came first and then ingenious development constructed the two cycle process.
The four cycle process involves the following:
The two cycle process involves:
The Four Cycle Engine
(Stroke 1) In the four cycle engine the air enters the cylinder by a glorified door or valve. The piston drops to the bottom of the cylinder correspondingly sucking air into the cylinder. The intake valve at the bottom of the stroke suddenly slams shut.
(Stroke 2)The compression event is where the piston now rises in the cylinder making the space in the cylinder much smaller, or decreasing the volume. This decrease in volume is like compacting air together. The more the air is compacted together the more efficiently or more powerfully the fuel can burn. So the aim in most engines is to make the compression really high. (but there is a limit…because the fuel will auto ignite, or preignite causing a smelly and poor burning of the fuel. It also will develop hot spots in the combustion chamber, and literally melt piston heads, valves, rings…ect)
(Stroke 3) The power event is where the fuel is suddenly ignited or burned. This sudden burning is started with a spark. If anyone has been around a stove, you will understand how a spark can cause immediate flames. A sudden “woof” and the gas is burning. The burning gas causes a corresponding sudden pressure rise and shoves on the piston face causing the piston to drive down the cylinder with considerable force.
(Stroke 4) Once the piston has reached the bottom of the cylinder, there is no more that can be mechanically done to extract more power, so the burnt mixture must be ushered out of the room or exit the cylinder with a glorified door, called and exhaust valve. The sudden “pow” or “bang” sound is caused by the remaining pressure that was developed by the combustion of the fuel. The harder you push on the gas pedal, the more fuel, the louder the bang.
Once the piston has reached the top of the cylinder, the exhaust valve slams shut and the intake opens to start the process all over again.
In the two cycle engine:
The intake and the compression even happen very close to each other. The complexity of the valving and the cams is reduced to a cylinder with holes in its sides. The holes in the sides of the cylinders is used to transfer gases in and out of the cylinder.
In a two cycle engine the fuel typically enters the crankcase and then is pushed into the cylinder by the pressure developed by the piston as it drops in the crankcase. Another way to look at it is that the piston serves two purposes. It acts as a power developer and also acts as a pump. On the downward stroke the piston serves as a pump and a power developer. On the upward stroke, the piston serves as a piston that sucks air/fuel into the crankcase.
So when the piston drops down the in the cylinder the air fuel mixture is pumped across the crown of the piston. This wave of fuel causes the remaining exhaust to be shoved out on the opposite side of the chamber to the exhaust port.
(Stroke 1) Now the cylinder goes up the cylinder, at the same time covering up both the intake ports and the exhaust ports. This is the compression event. At the top of the stroke the spark ignites the fuel causing a sudden temperature/pressure increase and the piston is fired down the cylinder.
(Stroke 2) As the piston approaches the bottom the ports are uncovered and the exhaust is shoved out of the cylinder by the intake charge that blows across the piston head.
The advantage of using a two cycle engine is that the power output is compact and light, but the down side is that the fuel consumption is generally poorer due to the fact that fuel can escape out with the exhaust and not get burnt.
Additionally, the parts of the engine are lubricated and cooled by the air/fuel/oil mixture as it passes over the parts. If the oil is not added to the fuel the engine will sieze up quickly more due to the piston not being able to be cooled down and lubricated. Once the piston gets hot it bakes off any lubrication causing metal on metal contact. The metal on metal will weld together or melt the rubbing surfaces. The engine will suddenly stop or sieze in this condition.
So needless to say, not having oil in the gas will kill a two cycle engine in short order.
The basic concept of an internal combustion engine is harnessing heat and converting it to useful pressure. This pressure is then harnessed by pistons or propellers (ie turbines).
The remaining complexity of an engine is harnessing the process as efficiently as possible and as compactly as possible. The basic concept of 4 cycle and 2 cycle still remain regardless of whether there is fuel injection, or computer aided engine management systems.
A simple carburetor and spark system will be around for a long time, so you can count on the basic concepts not changing in the near future on go kart engines.