What Is Air—Fuel Ratio

There are many factors that affect a car's power. The most important of which is the throttle control system. As we all know, the throttle system controls the ratio of intake air to fuel. Usually, when you step on the accelerator, you don't just add fuel, but you control how much intake air is used to influence the amount of fuel coming out of the injectors.

1. Introduction

2. Environment protecting

3. Adjustment

4. Conclusion

The ratio of air to fuel in the mixture is the air-fuel ratio. Specifically, it is the mixture's ratio between the air and the fuel. It is generally expressed in terms of the air quantity consumed per gram of fuel burned.

1. Introduction

The proper combustion process must have four crucial elements:

① The correct air content

② The correct fuel content

③ Air and fuel mixing in a sealed container

④ The mixture must be subjected to a sufficient amount of thermal shock at the right moment for combustion. 

So, the correct air-fuel ratio is necessary.

As we all know, the faster the combustion rate, the higher the combustion pressure will be, and thus the higher the engine output power.

It is found that when the air-fuel ratio at maximum combustion speed is around 12 to 13, the torque generated is the greatest. At this time, the engine has the best power performance. We call this classical ratio the power air-fuel ratio.

And when the air-fuel ratio is 15 to 16, due to the lean mixture, although the combustion rate is slow, it is conducive to complete combustion of fuel. This process can reduce fuel consumption. At this time, the fuel economy is the best. And we call it the economic air-fuel ratio.

Although we know the approximate range of the ideal air-fuel ratio, trying to control it is not simple. As the fuel does not cool the cylinder well, we need to make a series of adjustments to the engine above this ratio, which has led to the emergence of the lean-burn engine.

In 1996, Mitsubishi Motors was the first to introduce a modern electronically controlled in-cylinder fuel injection system into its 4G93 1.8L engine. 

A series of direct-injection types, such as the 6G74 and 4G15, went into service. In the later days, this technology became more and more popular.

The most important feature of lean combustion technology is its highly efficient, economical and environmentally friendly. The mixture ignition is more difficult to occur under lean combustion conditions than situations under theoretical air-fuel ratio. Deflagration is also less likely to occur.

As a result, a higher compression ratio can be designed to increase heat conversion efficiency. It is the fact that the fuel can be burned well in the excess air.

It is possible to squeeze all the energy out of all the fuel to support these conditions. Typically large air-fuel ratios of this type of engine can reach 25 or more, or even up to 65.

At low compression ratios, the power performance is not good. On the other hand, high air-fuel ratios help to improve fuel economy.

In order to balance the fuel economy and power performance, the most common approach is to obtain a high air-fuel ratio first and then increase the compression ratio to compensate for the lack of power.

A typical engine of this type is the Mitsubishi cylinder-injected petrol engine with an air-fuel ratio of up to 40. Honda's i-VTEC I direct-injection petrol engine has an air-fuel ratio of up to 65.

In recent years, lean combustion has become one of the buzzwords. For example, Mazda's SPCCI technology is part of the lean combustion technology that is sure to become one of the major developments for engines in the future.

2. Environment protecting

According to some studies, when a car is idling, the piston does not suck in enough mixture due to the low speed. The air-fuel ratio is low.

The fuel will have a surplus after the oxidizing reaction. It will continue to react with the carbon dioxide produced by combustion to produce carbon monoxide.

Generally speaking, when the air-fuel ratio is less than 16, the concentration of CO increases sharply as the air-fuel ratio decreases.

In fact, in addition to CO, the lean mixture also directly affects the production of hydrocarbons and nitrogen oxides (NOx). The emission trend for hydrocarbons is comparable to that of carbon monoxide, except that the rate does not change too much.

But NOx is in a different category. Neither too high nor too low is not good. The combustion efficiency is highest when the air-fuel ratio is around 15.5. In this situation, NOx production is at its maximum. The production decreases above or below this value.

Carbon dioxide (CO2) is an inevitable product of combustion. The closer the air-fuel ratio is to the theoretical air-fuel ratio of 14.7, the more complete the combustion and the higher the value of CO2.

3. Adjustment

The adjustment of the air-fuel ratio is quite tricky. For example, we want the engine to achieve complete combustion, but the production of nitrogen oxides will be high. Conversely, carbon monoxide and hydrocarbons production will increase when the combustion is incomplete.

What makes it even more difficult is that the engine is often in non-stationary transition conditions in the real world, such as cold starts, acceleration and deceleration.

Generally speaking, the air-fuel ratio is reduced during the cold start. This reduction ensures a smooth cold start. A high air-fuel ratio is required during the warm-up phase until the engine reaches normal temperature and runs steadily with a normal mixture.

When accelerating and decelerating, the air-fuel ratio should be reduced appropriately at the moment of acceleration and deceleration to obtain a good transition.

In short, we need different air-fuel ratios for different operating conditions in order to obtain the best engine running.

Since we need to control the air-fuel ratio, we naturally need to know what factors affect it, and we can usually divide them into three types:

①  Fuel film effect

Part of the fuel injected goes directly into the cylinder. Some of it will form a film on the inlet pipe wall. This process is mainly related to temperature and injection volume.

②  Sensor characteristics

The engine uses a particularly large number of sensors, including temperature, position, flow and pressure sensors. The accuracy of these sensors directly affects the accuracy of the air-fuel ratio control and the engine performance.

③  Delay time

Delay time is primarily the delay time of the engine controller. It generally includes injection delay time and transmission delay time.

In the carburetor era, the air-fuel ratio was adjusted by using the vacuum generated at the throat above the throttle to draw fuel out of the float chamber continuously. However, the accuracy and stability were not good.

Nowadays, it is mainly controlled by the vehicle's electronic control system. To adjust the air-fuel ratio, it is natural to have a device that can detect the oxygen content. As a result, the oxygen sensor is put into use.

We use this component to derive the actual air-fuel ratio for combustion at the time, which is then fed back to the ECU. The system uses built-in regulation logic to regulate a range of parameters such as injection volume and ignition timing in real time according to the required operating conditions. It controls the injector opening time for precise dosing.

There are two symptoms of automotive oxygen sensor failure: aging and contamination. The leading cause of aged sensor is the high local surface temperature of the sensing element.

The failure of an oxygen sensor due to contamination of the sensing element makes the engine not run smoothly.

The electronic control system is needless to talk about more. It mainly differs in its regulation strategy. However, for the driver, the oxygen sensor is a device that deserves the maintenance.

As aging and contamination are inevitable, the oxygen sensor should be replaced after driving a certain mileage. The sensor should also be checked regularly to see if it has failed and replaced in time to ensure safe driving and reduce fuel consumption and environmental pollution.

4. Conclusion

From the above analysis, we can see that changes in the air-fuel ratio, which are the most intuitive parameter of the mixture concentration, directly affect the engine performance.

We want the engine to achieve an optimum air-fuel ratio, which can effectively increase the engine power and improve the fuel economy. Thus it improves the engine performance.

Lauritz Carolsfeld        January 10, 2022

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