How to Select Combustion Engines
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Combustion engines are machines that use the heat and pressure from a combustion reaction to generate mechanical energy. Most combustion engines operate by inducing a controlled burn of fuel and air in a combustion chamber. The burn generates heat and pressure which directly or indirectly drives a shaft which does work. The mechanical energy produced by a combustion engine may be rotational, vibrational, or another form based on the design of the components. Combustion engines are incorporated in countless types of products, from automobiles to large industrial machines.
Types of Combustion Engines
Combustion engines are classified initially based on how they combust fuel (either internally or externally). Within each category, there are a number of different types of designs.
Internal Combustion Engines
Internal combustion engines are combustion engines which burn their fuel internally in a combustion chamber.
Two Stroke Engines
Two stroke engines complete the power cycle with two strokes of the piston within the cylinder, or one turn of the crankshaft. In these engines, the flow of the intake and the exhaust happen simultaneously.
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Often two stroke engines are labeled as simpler in design and have a higher power-to-weight ratios than four stroke engines. They also are considered to be less fuel efficient and more polluting. However, there are many exceptions to these generalizations, and performance varies greatly with different engine designs. Two stroke engines are used to generate power in a variety of applications, including small landscaping products (e.g. chainsaws, trimmers), power plant operations, and large ships.
Four Stroke Engines
Four stroke engines complete the power cycle with four strokes of the piston within the cylinder, or two turns of the crankshaft. In these engines, individual phases are separated, and intake and exhaust happen separately during the power cycle.
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CDX Textbook provides a great video that further explains four stroke engine operation.
Four stroke engines are often more fuel efficient and cleaner than equivalent two stroke designs, but may be heavier and more complex to design. They are the most common type of internal combustion engine, used in applications ranging from automobiles to industrial machinery.
Selection Tip: Theoretically, a two stroke engine can generate twice as much power as a four stroke engine for the same engine and the same number of revolutions. In reality, this is only nearly true for very large systems, where the power ratio is about 1.8:1. The average two stroke engine suffers power losses from a less complete intake and exhaust and shorter effective compression and power stroke, making the power output nearly equivalent.
Rotary (Wankel) Engines
Rotary (Wankel) engines operate using a rotor and shaft instead of a piston. The rotation of the shaft moves a three sided rotor which drives the movement of fuel through the system. In these engines, the different phases (intake, compression, power, and exhaust) take place in separate locations in the engine. The driveshaft rotates once for every time the engine fires in the Wankel design.
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Wankel engines are often lighter and simpler in design than equivalent piston engines. They are also typically more reliable (due to the reduction of moving parts) and have higher power-to-weight ratios. However, they suffer from less effective sealing which reduces their efficiency and lifespan. These engines are used mainly in racecars and sporting vehicles where reliability and lightness are considered more important than efficiency and engine life.
Jet engines are a subset of turbine engines that are optimized to produce thrust. To do work, hot gases generated by the combustion source are propelled through a nozzle at high speed. They are used as propulsion systems for aircraft.
External Combustion Engines
External combustion engines are combustion engines which burn their fuel externally, and use that heat to move an internal fluid which does the work.
Stirling engines are single-phase external combustion engines which use air, helium, or hydrogen as the working fluid. Every Stirling engine has a sealed cylinder with one part hot and the other cold. The working gas inside the engine is moved by a mechanism from the hot side to the cold side. When the gas is on the hot side, it expands and pushes up on a piston. When it moves back to the cold side it contracts. Properly designed Stirling engines have two power pulses per revolution, which can make them very smooth running. Stirling engines can reach much higher efficiencies than typical internal combustion engines, and produce less noise and vibration during operation. However, they cannot start running instantly like IC engines, which makes them less useful for applications such as vehicles and aircraft. They are most commonly used for heating, cooling, and underwater power systems.
Stirling Engine - Image Credit: MIT
Steam engines are two-phase external engines which use water (in liquid and vapor forms) as the working fluid. Steam engines can also use non-combustion heat sources such as solar power, nuclear power, or geothermal energy to heat the steam. Modern steam engines are used primarily in the form of turbines for generating electric power.
Combustion engines also vary based on the type of fuel they burn.
- Gasoline is a liquid fuel derived from petroleum (crude oil). Grades of gasoline differ based on octane rating (premium or "leaded" vs. regular or "unleaded"). Higher octane gasoline can withstand more compression before combustion, and is needed in some engines designed for higher compression to prevent knocking (uncontrolled combustion in the cylinder). Gasoline engines are also called spark ignition engines, meaning the fuel is burned by generating a spark from a spark plug in the cylinder.
Diesel is a liquid fuel made of long hydrocarbons derived from crude oil. Diesel has a high energy density and thus has better fuel economy (over 33% more efficient) than gasoline, but burns more dirty. Ultra-low sulfur diesel (ULSD) is a standard for diesel with low sulfur content; most grades of diesel fuel used today are ULSD. Diesel engines are compression ignition engines, meaning the fuel is burned by using compressed (high pressure) air to raise the temperature beyond the self-ignition (auto-ignition) point of the fuel. Because they do not use an ignition source (spark), diesel engines often require warming up under very cold conditions before use. Diesel engines also provide more torque than gasoline engines.
Liquefied propane gas (LPG) is a mixture of propane and butane which is a gas at standard conditions but can be stored and converted to a liquid at higher pressure. It can be used in IC engines as a gasoline (petrol) or diesel alternative that burns more cleanly, but has a lower energy density (meaning higher equivalent fuel usage). Some engines are not suitable for LPG because it provides less lubrication than other standard fuels, causing excessive valve wear within the cylinders.
Compressed natural gas (CNG) is mixture of methane and other hydrocarbons stored as high pressure gas. Natural gas is a relatively clean burning fuel with a lower energy density than gasoline and diesel. Natural gas engines are similar to standard gasoline or diesel engines; but they contain connectors which feed natural gas from storage cylinders and incorporate regulators to reduce the pressure. Like LPG, CNG does not provide the same amount of lubrication as standard liquid fuels, and engines must be designed and maintained appropriately to prevent valve wear.
Ethanol is an alcohol made from the fermentation and distillation of starch crops such as corn, or from cellulosic biomass such as switchgrass. Often ethanol is blended in conjunction with gasoline in amounts up to nine or ten percent (E10), though some engines can be designed to burn blends up to 85% pure ethanol (E85). Ethanol has a slightly lower energy content than gasoline, resulting in higher equivalent fuel consumption. However, ethanol emits fewer pollutants than gasoline, and also has more resistance to engine knock than gasoline.
Jet fuel is a mixture of variety of different hydrocarbons. It is used specifically for gas turbine engines and jet engines used for aviation applications. Mixtures differ based on the properties required for the product. Turbine and diesel engines used to power aircraft use kerosene-based jet fuel, while aircraft with piston or Wankel engines use what is called avgas (aviation gasoline).
Other fuels that can be used to power certain types of engines include vegetable oil, hydrogen, butane, and wood (via gasification).
The most important specifications to consider when selecting combustion engines are torque, horsepower, and RPM (shaft speed), which are all interdependent. For internal combustion engines, displacement and the number of cylinders are also important to consider.
Torque (τ) is a measure of the rotational force generated on engine's shaft during the power stroke, given in distance-force units (ft-lb, in-lb, m-N, etc.). It defines the amount of physical load the engine can generate. The torque specification typically is an indication of the engine's maximum rated torque according to SAE standards. Torque measures an engine's ability to handle loads and accelerate, and is perhaps the best indicator of an engine's performance. Engines produce useful torque only over a limited range of rotational speeds (discussed below). Optimal use of an engine's torque is often largely dependent on the gearing of the associated system's drive train.
Selection Tip: It is important to check the standards that the manufacturer uses to base its torque measurement. Advertised ratings not based on certain standards may be deceiving and not accurate.
RPM or shaft speed is the speed at which the shaft, disc, or rotor in the engine rotates measured in rpm (rotations per minute). Because speed and torque are interdependent, RPM ratings for engines often define the speed at which maximum torque occurs. Automobile engines typically run at speeds around 2500 rpm. Stalling occurs when engines run below the minimum speed, and damage or failure can occur when running above the recommended maximum. Engines that run at lower speeds may last longer than equivalent motors at higher speeds because they complete less cycles and wear less over time. In vehicles, RPM is measured by a tachometer.
Horsepower (hp) is a derived specification which indicates an engine's performance. Specifically, it defines the rate of energy transfer in the engine. Like torque, rated horsepower is given over a range of different engine speeds. Horsepower is a function of engine speed and torque by the equation:
hp = (τ×rpm)÷5252
hp is the horsepower
τ is the torque in ft-lb
rpm is the speed in rpm
5252 is the unit conversion factor.
Here is a simplified example of what torque and power curves will look like for a small IC engine:
Engine power and torque performance curves. Image Credit: Woodbank Communications Ltd
The power and torque increase with engine speed and reach a peak as physical limitations begin to take effect. These limitations include the intake and exhaust path size/shape, fuel mixing efficiency, flame propagation rate, friction, and mechanical component strength.
Displacement is the volume displaced by all the pistons in an internal combustion engine during one stroke. It is typically measured in cubic centimeters (cc), cubic inches (CID). Displacement is a basic part of engine design which determines how much fuel can be injected or mixed in the cylinder during each power cycle. This has a significant effect the maximum power that the engine can produce.
The number of cylinders describes the amount of combustion cylinders in an internal combustion engine. The number of cylinders in an engine directly affects the amount of power produced, since more cylinders means more fuel combustion and more power strokes. Engines with more cylinders will, as a result, consume more fuel than those with less.
Other Engine Specifications
In addition to the key performance specifications, there are a number of other engine specifications and parameters for buyers to consider.
Fuel consumption - Fuel consumption defines the amount of fuel consumed. Like torque and horsepower, fuel consumption changes based on the engine's speed. It is often specified by manufacturers as a range of values on a performance curve.
Engine efficiency - Energy efficiency describes the amount of energy from the fuel used by the engine to do useful work. For gasoline engines, maximum efficiencies typically range between 25-30% since 70-75% is lost as unused heat energy. More efficient engines will have better fuel economy (i.e. lower overall fuel consumption).
Emissions - Gaseous emissions of pollutants and particulates are released in the exhaust streams of combustion engines after the fuel is burned. The makeup of this exhaust is important to consider when complying with pollution and emission standards and requirements. Factors effecting exhaust emissions include the composition of the fuel and the combustion conditions (e.g. air-fuel ratio, whether the fuel burns completely).
Weight - The weight of the engine is important in terms of portability and placement. Lighter engines are ideal for applications where the powered system must be portable or involves transport, since heavier systems require more torque to move. For stationary applications, weight is often less of an issue.
Dimensions - The dimensions of the engine must fit within the requirements of the corresponding system or environment. Dimensions include the width, length, and height of the engine.
Compression ratio - The ratio of an engine's combustion chamber volume at its largest to the volume at its smallest. It defines how much compression takes place within the chamber. A high compression ratio results in better fuel-air mixing and ignition, which leads to increased power and better overall engine efficiency. However, higher compression ratios make engines more susceptible to knocking with lower octane fuels, which can reduce efficiency or cause damage.
There are a number of parameters that define different engine requirements which need to be considered during selection.
Air requirements - The quality or makeup of air used in the engine to mix with the fuel during combustion. While most engines run using standard ambient air, certain environments may require the use of filters to remove particulates or undesirable gases from the air.
Cooling requirements - Engines require cooling to remove the waste heat that is generated during operation. Internal combustion engines are cooled with either air or liquid. Air cooled engines can operate over a larger range of temperatures than some liquid cooled engine because air is not subject to freezing or boiling. However, liquid cooled systems are often more flexible to the cooling needs of different parts of the engine, reducing hot spots and large temperature differentials. Today, most combustion engines are liquid-cooled.
Oil requirements - Engines require lubrication to keep the moving parts from excessive wear during operation. Oil is used to provide this lubrication, put either in an independent system or directly mixed with the fuel being combusted. Different engines require different grades of oil and lubricant for proper operation and maintenance. In addition, because lubricants become dirty and degrade over time, they must be replaced regularly after a certain number of cycles or hours of operation.
Combustion engines are designed with a number of different features which may be important to consider in the selection process.
Carbureted engines are engines which incorporate carburetors, designed to blend the air and fuel mixture in the combustion chamber. Carburetors use suction created by intake air moving through a venturi to draw fuel into the airstream. Compared to fuel injectors, carburetors are much easier to adjust, fix, and rebuild. They also cost less than fuel injection systems, and are more reliable.
Fuel injected engines are engines which incorporate fuel injectors, designed to deliver fuel to the combustion chamber. Fuel injectors atomize fuel into droplets in the chamber by forcing it through a nozzle at high pressure. They rely on computers which constantly change air-to-fuel ratios for optimization. Compared to carburetors, fuel injectors are more precise and efficient, and less polluting.
Turbo charged engines are those which incorporate turbochargers designed to boost the combustion engine's efficiency. Turbochargers are found most commonly alongside gasoline and diesel IC engines.
Flex fuel or multi-fuel engines are designed to be compatible with multiple different types or blends of fuel. For example, a spark ignition engine for an automobile may be able to run on different blends of gasoline with up to 85% ethanol, or may have added components to be able to burn compressed natural gas.