What Is The Maximum Efficiency Of An Internal Combustion Engine?

What is an internal combustion engine?

An internal combustion engine (ICE) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber, producing hot gases that expand to move a piston or turbine blade. The expansion of the high-temperature and high-pressure gases directly applies force to components of the engine, like pistons, turbine blades, or a nozzle. This force moves the components over a distance, generating useful mechanical energy. Internal combustion engines convert chemical energy in the fuel into mechanical work.

The combustion is intermittent, with the ignition of the fuel at regular intervals and separate expansion in the combustion chamber. This contrasts with external combustion engines like steam engines and Stirling engines, which use an external combustion chamber to burn a fuel but use the heated working fluid from that reaction to produce mechanical work through the expansion of the fluid.

Some key advantages of internal combustion engines are high power-to-weight ratio, smaller size, and adaptability to various fuels. These make them dominant for vehicles and portable applications today. Disadvantages include air pollution emissions, reliance on fossil fuels, lower efficiency than external combustion engines, and noise pollution.

Internal combustion engines can deliver power in the range from 0.01 kW to 20×106 kW, depending on their displacement. They are used for small, portable applications like chainsaws and for extremely large applications like container ships. They can provide propulsion for vehicles ranging from motorcycles to supertankers and locomotives.

The most widespread form of internal combustion engine is the four-stroke Otto cycle petrol engine, used in cars and trucks. Diesel engines are used in larger vehicles and provide higher torque at lower revolutions per minute (RPM). Other design types include rotary engines like the Wankel engine and two-stroke engines like those used in dirt bikes.




Thermodynamic limits

The maximum possible efficiency of any heat engine is defined by thermodynamic principles, specifically the Carnot cycle. The Carnot cycle sets an absolute maximum on the fraction of heat energy that can be converted into useful work via a heat engine. This efficiency limit is based on the difference in temperatures between the hot and cold reservoirs that supply and absorb heat from the engine.

For an ideal Carnot engine operating between a hot reservoir at temperature TH and a cold reservoir at temperature TC, the maximum possible efficiency is:

ηth,max = 1 – TC/TH

Where ηth,max is the maximum thermal efficiency. This efficiency depends only on the temperature difference between the hot and cold reservoirs.

An internal combustion engine, operating between the temperatures of the combustion gas and the ambient cooling system, is subject to this same efficiency limit. The maximum possible thermal efficiency dictates the fraction of heat from fuel combustion that can be converted into usable work output.

For more details on thermodynamic limits see: Maximum theoretical efficiency of internal combustion engine

Otto Cycle

The Otto cycle is a thermodynamic cycle commonly used in spark ignition internal combustion engines such as gasoline engines. It was developed by the German engineer Nikolaus Otto in 1876. In the Otto cycle, a piston moves up and down inside a cylinder in four stages: intake, compression, power, and exhaust.

During the intake stroke, the intake valve opens and a fuel-air mixture is drawn into the cylinder as the piston moves down. In the compression stroke, the intake valve closes and the piston moves up, compressing the fuel-air mixture. Near the end of the compression stroke, a spark plug ignites the compressed fuel-air mixture. The resulting combustion rapidly increases pressure, pushing the piston down during the power stroke and turning the crankshaft. Finally, during the exhaust stroke, the exhaust valve opens and the hot combustion gases are pushed out as the piston moves back up. This four stroke sequence is repeated continuously in the engine.

The thermodynamic efficiency of the Otto cycle depends on the compression ratio of the engine. Higher compression ratios result in higher efficiency but also higher combustion temperatures. Gasoline engines typically have compression ratios in the range of 8:1 to 12:1. The theoretical maximum efficiency of the Otto cycle is about 65-70% (“Thermodynamic limits,” n.d.). However, actual engine efficiencies are much lower, around 20-30%, due to heat and friction losses as well as throttling losses from not running the engine at full throttle (MIT, n.d.).
diagram showing the four stages of the otto cycle in a gasoline engine

Diesel cycle

The Diesel cycle is a thermodynamic cycle commonly used in diesel engines. It was invented by German engineer Rudolf Diesel in the 1890s. Compared to the Otto cycle used in gasoline engines, the Diesel cycle is more efficient due to its higher compression ratio and ability to operate without throttling losses.Diesel cycle

In the ideal Diesel cycle, air is drawn into the cylinder and compressed adiabatically with a compression ratio typically between 15:1 and 20:1. Fuel is then injected and ignited. The resulting combustion increases the temperature and pressure further. This is followed by an adiabatic expansion as the piston moves down, harnessing useful work. Finally, the exhaust stroke expels the high-pressure air.

This thermodynamic cycle provides higher efficiency because compression ratios are higher and there is no intake restriction like a throttle plate. Thermal efficiency for an ideal Diesel cycle can reach over 50%. However, in real engines, mechanical and combustion inefficiencies lower the maximum efficiency to around 35-45%. Still, this makes the Diesel engine the most efficient practical internal combustion engine.

Factors affecting efficiency

There are several factors that affect the maximum efficiency of an internal combustion engine. The two main ones are combustion efficiency and mechanical losses.

Combustion efficiency refers to how much of the fuel’s chemical energy is converted into heat energy during combustion. According to Dieselnet.com, “These include the chemical energy loss in emissions, heat losses from the engine and through the exhaust gas, and gas pumping and friction losses in the engine.” [1] Maximizing combustion efficiency involves optimizing the air-fuel mixture, compression ratio, combustion chamber design, and more.

Mechanical losses occur due to the friction between moving engine components. This includes friction in the pistons, bearings, valves, and other parts. Mechanical friction converts useful energy into heat which is dissipated and wasted. Minimizing friction can increase efficiency.

Another factor is exhaust heat. A significant amount of heat is lost through the hot exhaust gases leaving the engine. Methods like turbocharging, exhaust gas recirculation, and waste heat recovery can be used to recover some of this wasted heat energy.

Current gasoline engine efficiency

The typical thermal efficiency of modern gasoline engines in passenger cars is around 20-30%. According to the U.S. Department of Energy, the average efficiency of gasoline engines in light-duty vehicles was 20% in 2016 [1]. This means that only 20-30% of the chemical energy in the gasoline is converted into usable mechanical work to move the vehicle.

There are several factors that limit the efficiency of gasoline engines. One is the compression ratio, which is the ratio between the volume of the combustion chamber from its largest capacity to its smallest capacity. Higher compression ratios result in more efficient combustion but increase the risk of premature ignition or engine knock. Most gasoline engines have compression ratios of around 10:1, as higher ratios would require higher octane fuels [2].

Another factor is heat loss during combustion and exhaust. A lot of the chemical energy in the fuel ends up heating the cylinder walls rather than pushing the pistons. Improving insulation around combustion chambers can help reduce this waste heat loss. Gasoline engines also lose energy out the exhaust as hot gases escape the cylinders after each combustion stroke [3].

Current diesel engine efficiency

Typical efficiencies of modern diesel engines range from 42-50% for large stationary engines up to 55% for automobile engines. According to Volvo (“https://www.volvoce.com/global/en/news-and-events/news-and-stories/2018/fuel-use-how-low-can-you-go/”), modern diesel engines in tractor-trailers can operate at peak efficiencies over 50%. Cummins, a major diesel engine manufacturer, reports that their engines will see improvements in fuel efficiency of 9-15% by 2017 compared to previous models (“https://theicct.org/the-ever-improving-efficiency-of-the-diesel-engine/”).

Many factors affect diesel engine efficiency, but improvements in fuel injection, turbocharging, combustion chamber design, and electronic engine controls have allowed the diesel engine to become increasingly more efficient in recent decades. While early diesel engines had brake thermal efficiencies of around 20%, current engines can exceed 45% efficiency.

Future improvements

There are several emerging technologies that can potentially improve ICE efficiency further. One area of research is lower temperature combustion, where engines are designed to run on more diluted fuel mixtures and operate at lower peak combustion temperatures to reduce heat losses and increase efficiency (https://www.sciencedirect.com/science/article/pii/S2666691X20300063). Other techniques like exhaust gas recirculation, variable compression ratio, and lean burn combustion also aim to improve efficiency.

Engine downsizing paired with turbocharging or supercharging allows similar power outputs from smaller displacement engines operating at higher loads, improving efficiency (https://courses.washington.edu/me341/oct22v2.htm). Direct injection fuel systems and new combustion chamber designs provide more control over the combustion process as well. Advanced materials like ceramics and carbon composites can reduce weight and improve thermal efficiency.

Overall, incremental improvements in existing technologies as well as new engine designs and architectures will continue to push the limits of ICE efficiency in the future.

Electric vehicles

In contrast to internal combustion engines, electric motors are much more efficient at converting input energy into useful work. According to the NRDC, electric motors convert over 85% of their electrical input energy into mechanical energy to drive the wheels, compared to about 15-30% for gasoline engines [1]. This fundamentally higher efficiency allows electric vehicles to travel much farther using the same amount of input energy.

For example, a typical electric vehicle can travel about 3 miles per kWh of battery energy. A comparable gasoline-powered car would only manage around 0.5 miles per kWh if running on an internal combustion engine [2]. So EVs can go around 6 times farther using the same amount of raw energy, thanks to their more efficient electric motors.

This enormous efficiency advantage is why electric vehicles offer such compelling environmental and economic benefits compared to traditional internal combustion engine vehicles.

Maximum possible efficiency

The theoretical maximum efficiency of an internal combustion engine is determined by the thermodynamic limit of the Otto cycle. This limit is based on the specific heat ratio of the working fluid and the compression ratio. For an ideal Otto cycle operating with air as the working fluid, the maximum efficiency is:

ηmax = 1 – 1/r(γ-1)

Where r is the compression ratio and γ is the specific heat ratio (about 1.4 for air). With a compression ratio of 10:1, which is typical for gasoline engines, the maximum theoretical efficiency is about 56%. With a compression ratio of 18:1, the maximum theoretical efficiency increases to 63%.

However, real-world engines have efficiencies much lower than this theoretical limit due to irreversibilities such as heat loss to the cylinder walls, friction, incomplete combustion, throttling losses, and exhaust blowdown. The highest efficiency achieved in practice for a production multi-cylinder piston engine is around 45% for large diesel engines. Smaller automobile engines have peak efficiencies in the 30-40% range.

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