Why Do Combustion Engines in Cars Operate at Low Efficiency?

Why Do Combustion Engines in Cars Operate at Low Efficiency? image 0 Engine power

The best combustion engines convert around forty percent of the energy in fuel into movement. The rest is lost through friction and heat. This means you spend $100 on gas and get only about $60 in action. Most combustion engines are less efficient than this. Despite this, some models do have impressive fuel efficiency.

Expansion ratio

Combustion engines have various compression ratios. The higher the compression ratio, the greater the energy the machine can extract from the fuel. Higher compression ratios also result in higher efficiency. They can achieve the same combustion temperature using less fuel. This gives the engine more motive power. Higher compression ratios also require a higher octane fuel, as lower-octane fuels are more likely to combust under pressure spontaneously.

Various studies have shown that a vast expansion reciprocating piston engine offers passenger cars the most significant efficiency potential. In a recent survey, three prototype engines were constructed and measured on a testbed, and thermodynamic simulations were used to validate the results. The results showed that a machine with an expanded expansion ratio of g = 1.56 achieves up to 7 percentage points of efficiency improvement over a conventional crank train. The vast expansion also has no adverse impact on combustion, emission formation, or wall heat transfer.

The compression/expansion ratio is usually around nine to ten in a conventional car engine. It’s important to note that these numbers are theoretical and that real machines are often not what they appear on paper. For example, the exhaust valve in a high-performance race car may open ninety degrees at ABDC to clear the cylinder and maintain a constant volume. But these ratios do not matter when making power, as the volumetric and fuel conversion efficiency are more important factors.

Compression ratios refer to the difference between the maximum and minimum cylinder volumes. The total cylinder volume occurs when the piston is at the bottom dead center of the cylinder, while the minimum volume occurs when the piston is at the top dead center. Moreover, the minimum cylinder volume is at the top dead center.

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Compression ratios are derived from the pressure of the piston at the bottom dead center (BDC) and the pressure of the cylinder at TDC. These two measurements are then combined to get the overall compression ratio. Overall volume is then derived from the compression ratio and the crank train expansion ratio.

Compression ratio

The compression ratio is the percentage of air volume in a cylinder at the bottom dead center compared to the importance of the cylinder at the top dead center. For example, a 10:1 compression ratio means that the air volume in a cylinder equals the amount of air in the cylinder. The cylinder head design is another critical factor in determining the compression ratio. If the cylinder head is too large or too small, the engine will not operate as efficiently.

A higher compression ratio produces more horsepower and greater efficiency. A higher compression ratio permits the intake and exhaust ports to overlap longer. This results in more air/fuel mixture used in the combustion chamber. High compression also produces more torque, which results in greater horsepower. In addition, the compression ratio directly correlates with power output. A higher compression ratio caused the engine to knock in the past, but today’s modern engines have knock sensors to avoid this problem.

The compression ratio of combustion engines in cars is generally low. Modern cars increase the compression ratio by replacing carburetors with fuel injectors, doubling the number of valves per cylinder, and using turbochargers to mix the air and fuel mixture before it enters the cylinder.

Higher compression ratios also increase fuel efficiency. High compression ratios allow higher-octane fuel to be used, but the disadvantage of a high-compression ratio is high fuel prices. Additionally, the higher octane gas prevents engine knock, which is caused when the air-fuel mixture does not burn at the optimal moment.

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Combustion efficiency varies greatly depending on the type of fuel. Fuels with a high LHV are the most efficient, while those with low LHVs are less efficient. The same principle applies to hydrocarbon fuels. Fuels with oxygenated functional groups contribute less net energy during combustion but donate a considerable portion of the fuel’s volume and mass.

Diesel fuel is naturally more efficient than gasoline. The higher diesel fuel density means it can burn more energy per unit. That means a diesel engine can reach a higher compression ratio than a gasoline engine. In addition, diesel fuel has higher thermal efficiency.


The combustion chamber of an automotive engine is a hot place. The burning gases inside the room reach a temperature of approximately 2,800deg F. This temperature is not safe for metal parts of the machine. Steel melts at 2500deg F, while aluminum alloys melt at 1200deg F. The burning gases’ heat can deform the engine’s metal parts, causing catastrophic failure.

C combustion engines in cars operate at low efficiency primarily due to temperature. The higher the temperature, the more the combustion phasing is advanced. Higher temperature leads to higher charge temperatures during compression, which promote auto-ignition. The combustion process can also result in knocking and damage to engine components.

The efficiency of combustion engines is inversely proportional to the fuel used. A typical gasoline engine converts only about half of its fuel energy into mechanical energy. The remaining one-third of the fuel energy is lost in the form of heat. This heat escapes the vehicle via the exhaust and is released into the environment.

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The first law of thermodynamics

The First Law of Thermodynamics describes the basic energy exchange process between objects. During this process, matter undergoes a change in energy, known as internal energy, and that change is represented by heat. The power shift occurs only when an object produces or receives work. This is why combustion engines in cars are so inefficient.

Thermal efficiency is the ratio of heat energy input to the amount of heat energy output. As a result, the production of an engine can never be greater than the input of heat energy. Similarly, the heat energy consumed by a combustion engine cannot exceed the energy stored in the fuel. This is because the efficiency ratio cannot be higher than 100 percent.

While thermal efficiency is difficult to achieve, some car manufacturers are taking steps to increase it. The average gasoline combustion engine is less than 50 percent efficient. On the other hand, diesel engines produce more energy per unit volume than gasoline engines. Even so, matching older diesel engines’ thermal efficiency is impossible.

The compression ratio is a critical parameter in determining the thermal efficiency of combustion engines in cars. A typical car engine’s compression ratio equals (V max / V min) x (V min). The compression ratio of a gasoline engine should be less than 10:1 or less than six.

Despite the improvements in car fuel efficiency, combustion engines remain uneconomic. Most vehicles have an energy efficiency of 25% to 30%, which means that only a fraction of the gas is converted to kinetic energy. This means that only a portion of the gas is converted into kinetic energy, which is the car’s power moving forward.

While we can’t directly measure the fuel efficiency of cars, we can observe a few examples of their low efficiency. Diesel engines, for example, have a high compression ratio and spontaneously release heat energy. They are still the machines of choice for heavy vehicles.

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