Mercedes Mullins
The air-fuel ratio (AFR) is an important factor in the functioning of an engine. Gasoline engines generally operate within a range of about 12 to 18 parts air to 1 part fuel (by mass), with a typical ratio of 14.7:1. In contrast, diesel engines have a much wider range of AFR, typically operating from 18:1 to as high as 70:1 and are capable of operating in super lean ratios. This is because diesel engines inject fuel into the cylinder after the air is compressed, allowing for higher compression ratios and, consequently, higher efficiency. The accelerator pedal in a diesel engine regulates how much fuel is injected, and pressing it drops the AFR, increasing horsepower.
| Characteristics | Values |
|---|---|
| Air/Fuel Ratio Range | 18:1 to 70:1 |
| Air/Fuel Ratio During Acceleration | 18:1 to 16.5:1 |
| Air/Fuel Ratio During Steady State Lug Curve | 28:1 (rated) to 22:1 (peak torque) |
| Air/Fuel Ratio at Idle | 50 to 65:1 |
| Compression Ratio | 14:1 to 24:1 |
| Peak Combustion Temperature | 15.5 to 17:1 |
| Fuel Injection Timing | 24º to 26º before TDC or at TDC |
| RPM Range | Below 4000 RPM |
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What You'll Learn
Air throttle absence in diesel engines
Diesel engines are throttled differently from gasoline engines. In a gasoline engine, pressing the accelerator pedal opens a valve in the engine's intake, allowing more air to enter the engine and creating more power. The engine's computer then injects more fuel to match the additional air.
Diesel engines, on the other hand, do not require a throttle body or throttle plate for efficient operation. They compress air until it becomes superheated and only then inject fuel directly into the combustion chamber. The heat of compression is enough to ignite the fuel, eliminating the need for spark plugs or precise air control via a throttle body. This unrestricted air intake also reduces vacuum pressure losses.
The absence of a throttle body in diesel engines offers several advantages. Firstly, it allows for higher fuel efficiency, torque, and longer lifespans than gasoline engines. Secondly, it enables diesel engines to operate within a very wide range of air-fuel ratios, typically between 18:1 to 70:1, with some sources stating ratios as high as 90:1 at idle. This flexibility in air-fuel ratios contributes to the fuel efficiency of diesel engines.
While most diesel engines lack throttle bodies, some modern diesel engines have implemented throttle controls and valves for emissions control and smoother engine shutdown. These valves restrict intake airflow, helping to increase exhaust gas recirculation rates and reduce nitrogen oxide emissions.
In summary, the air throttle absence in diesel engines is a key differentiator from gasoline engines and contributes to their higher fuel efficiency, wider operating range of air-fuel ratios, and overall unique characteristics.
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Direct fuel injection
The air-fuel ratio (AFR) in diesel engines can vary greatly, and typically operates within a range of 18:1 to 70:1, with a peak of 18:1 to 16.5:1 during acceleration. This is in contrast to gasoline engines, which generally operate within a range of 12 to 18 parts of air to 1 part of fuel, usually staying close to a ratio of 14.7:1.
Diesel engines inject fuel into the cylinder after the air is compressed, allowing for higher compression ratios and greater efficiency. This design also prevents predetonation, which can cause catastrophic damage to gasoline engines.
Now, let's focus on direct fuel injection in diesel engines:
Direct injection, also known as DI, is a fuel injection system that delivers fuel directly into the combustion chamber within the piston. DI diesel engines operate with higher injection pressure, which can exceed 30,000 psi in modern engines. This high pressure results in increased horsepower and torque gains, allowing the engine to be more powerful, efficient, and cleaner. The higher pressure also contributes to better thermal efficiency, and most DI engines are turbocharged.
The first successful prototype of a direct-injected diesel engine was created in 1894, and the first direct-injected diesel engine for trucks was introduced by MAN Truck & Bus in 1924. Higher-pressure diesel injection pumps were later developed by Bosch in 1927.
Direct injection's advantages come at a cost, as the higher pressure requirements call for more expensive injection pumps, injectors, and fuel lines. In contrast, indirect injection (IDI) diesel engines have lower pressure requirements, resulting in lower costs and a longer service life for vital components. IDI engines also have simpler injection pump and injector designs, making them suitable for alternative fuels such as waste vegetable oil and biodiesel.
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Compression ratios
A high compression ratio is desirable because it allows an engine to extract more mechanical energy from a given mass of air-fuel mixture due to its higher thermal efficiency. This occurs because internal combustion engines are heat engines, and higher compression ratios permit the same combustion temperature to be reached with less fuel, while giving a longer expansion. The higher the compression ratio, the more power is generated.
Diesel engines use higher compression ratios than petrol engines. A typical compression ratio for diesel engines is 14:1 to as high as 25:1, with a common range of 18-22. Their gasoline counterparts will typically have compression ratios between 8:1 and 12:1. The higher compression ratio of the diesel engine leads to better efficiency. In the case of a diesel engine, self-ignition is desired and part of the concept, whereas in a gasoline engine, it is harmful.
Diesel engines use direct fuel injection, meaning diesel fuel is injected directly into the cylinder. A diesel engine compresses only air, so the compression ratio can be much higher. The higher compression ratio must increase the temperature of the air in the cylinder sufficiently to ignite the diesel fuel using compression ignition.
Some modern diesel engines have lower compression ratios to help reduce nitrogen oxide emissions. Engines with turbochargers or superchargers also have lower compression ratios because the air is compressed by the turbocharger or supercharger before it enters the cylinders.
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Self-ignition temperature
The self-ignition temperature, or autoignition temperature, is the temperature at which an air-fuel mixture will combust without an external ignition source, such as a spark plug. In other words, it is the temperature at which the fuel will spontaneously ignite.
In the context of diesel engines, the self-ignition temperature is an important factor in their unique operational characteristics. Diesel engines do not mix fuel with air during the intake stroke, unlike gasoline engines. Instead, they compress the air first and then inject the fuel directly into the cylinder when it is ready to fire. This allows diesel engines to exceed the self-ignition temperature of diesel fuel during the compression stroke without causing predetonation. Predetonation occurs when the fuel mixture ignites before the spark plug fires, which can cause catastrophic damage to the engine.
The self-ignition temperature of diesel fuel is approximately 210 °C (410 °F), while petrol has a higher self-ignition temperature of 280 °C (536 °F). This difference in self-ignition temperatures contributes to the distinct characteristics of diesel and gasoline engines.
It is important to note that the self-ignition temperature can vary due to several factors, including partial oxygen pressure, altitude, humidity, and the amount of time required for ignition. These factors can influence the autoignition temperature of hydrocarbon/air mixtures, causing variations in the literature. Therefore, the temperatures should be used as estimates rather than absolute values.
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Air/fuel ratio range
The air-fuel ratio in an engine is the ratio of air to fuel in the cylinder. In a gasoline engine, the air and fuel are mixed before entering the cylinder, whereas in a diesel engine, only air goes into the cylinder, and the fuel is injected after the air is compressed. This design difference means that gasoline engines have a relatively low compression ratio to prevent self-ignition before the spark plug fires, which could cause catastrophic damage.
Diesel engines, on the other hand, can operate within a very wide range of air-fuel ratios, from 18:1 to as high as 70:1, and are capable of operating in super lean ratios. At idle, a diesel engine can have an air-fuel ratio of 90:1, compared to 14:1 for a gasoline engine. During a steady-state lug curve, a diesel engine typically runs at ~28:1 (rated) to ~22:1 (peak torque) and 50 to 65:1 at idle.
The peak combustion temperature for both gasoline and diesel engines is around 15.5 to 17:1; anything richer or leaner will result in a cooler combustion temperature. Going richer than about 22:1 to 25:1 in a diesel engine produces excessive temperature, soot, smoke, and poor fuel economy.
The air-fuel ratio in a diesel engine is controlled by the accelerator pedal, which regulates how much fuel is injected. When the accelerator is pressed, the air-fuel ratio drops, and more fuel is injected relative to the air ingested, increasing horsepower.
The ability of diesel engines to operate within a wide range of air-fuel ratios and at higher compression ratios contributes to their impressive fuel economy and efficiency compared to gasoline engines.
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Frequently asked questions
Diesel engines generally operate from 18:1 to as high as 70:1, and are capable of operating in super lean ratios.
The ideal fuel/air ratio for a TDI diesel engine is not clear, but one source mentions that the VW Sidewinder injects fuel beginning 24º before TDC on the compression stroke.
Gasoline engines generally operate within a range of about 12 to 18 parts air to 1 part fuel (by mass). Usually, this ratio stays pretty close to 14.7:1. Diesel engines have a much broader range, from 15:1 to as lean as 60:1, but going richer than about 22:1 to 25:1 produces excessive temperature, soot, smoke, and poor fuel economy.
Diesel engines do not have fuel in the mixture during the intake stroke, so they can compress the air much more, exceeding the diesel fuel’s self-ignition temperature. Higher compression ratios equate to higher efficiency.
