Engines produce power by combusting air and fuel inside the chambers, but these substances must be at the correct amounts in what we refer to as the air-to-fuel ratio.
The optimal air-fuel ratio for a car engine is 14.7:1 – anything too low or too high will prevent the engine from functioning correctly. So, what’s so special about these numbers?
A car engine must operate with the optimal air-fuel ratio of 14.7:1, meaning, there should be 14.7 units of air relative to 1 unit of fuel. Too much air will result in a lean mixture, while too little air makes the mixture rich. Both conditions can be problematic for an engine in the long run.
In this article, you’ll learn everything you need about the air-fuel ratio in combustion engine vehicles.
Let’s get started.
What Is the Air-to-Fuel-Ratio?
You’ve probably heard this said many times before. A combustion engine needs three things to work – air, fuel, and a spark.
Many people don’t know there is much more air than fuel in any combustion mixture. Moreover, the relative amount of air compared to fuel must be a specific amount to be considered optimal, and that relationship is called the air-to-fuel ratio.
The objective of having an optimal air-to-fuel ratio is so that each combustion burns all the fuel in the mixture without leaving any excess air. When that happens, the engine functions optimally to produce a stable energy output without generating excessive emissions.
That ratio is typically measured automatically by your car’s onboard computer. Various sensors in your vehicle supply real-time data to the computer so it knows the current air-to-fuel ratio.
An example is the oxygen or O2 sensor in your car’s exhaust system. It measures how much oxygen is coming out of the engine after combustion to determine whether there’s excess air exiting the engine.
The onboard computer can then use that data to make changes and optimise the balance of air and fuel entering the engine.
If you’d like to know your engine’s current ratio, you can find out by using an air-to-fuel ratio gauge that you’ll have to connect to the engine.
What Is a Good Air-to-Fuel Ratio?
Generally, the optimal air-to-fuel ratio for an engine is 14.7:1. That means there should be 14.7 parts air for every 1 part of fuel. That ratio provides enough air to burn all the fuel in the mixture without leaving any excess air.
However, you must also understand that what's considered 'optimal' differs depending on the driving condition. If you have access to air-fuel ratio gauges, you can test your engine in different scenarios to ensure it will function optimally in all situations.
Here are the optimal air-to-fuel ratios for different situations:
- Starting the engine: 9:1 ratio
- Idling: 12:1 ratio
- Accelerating: 13:1 ratio
- Cruising speed: 14.7:1 ratio
- Heavy load (uphill or towing): 12:1 ratio
- Decelerating: 17:1 ratio
As you can see from the list above, the optimal or ideal air-to-fuel ratio depends on the energy demand placed on the engine.
High-demand scenarios, like towing a heavy load, require a mixture with the most fuel (rich mixture), which produces more power. Meanwhile, light driving conditions, like descending or decelerating, require less fuel and more air (lean mixture), which generates less power.
In the next section below, you’ll learn more about the differences between rich and lean air-fuel mixtures and how they occur.
What Does Rich Or Lean Air-Fuel Ratio (AFR) Mean?
As you read above, monitoring your engine under different conditions using an air-fuel ratio gauge will give you different readings. That’s because the ratio changes based on the demand you place on the engine.
Still, there are scenarios where an engine will burn an incorrect air-fuel ratio due to a fault with the vehicle’s components.
For example, the air-fuel ratio is called ‘rich’ when there is less air than the optimal ratio. When that happens, the engine will produce more power. Still, it will also produce excess emissions and cause a drop in fuel economy.
Meanwhile, the air-fuel ratio is considered ‘lean’ when too much air is in the mixture. As a result, the engine will produce less power, though there will be an improvement in fuel economy and emissions.
Earlier, you saw that different driving conditions use a rich or lean air-fuel ratio. But if that happens when it shouldn’t, the vehicle is not functioning correctly.
What Causes a Rich Air-Fuel Mixture?
When your car’s engine runs on a rich air-fuel mixture regardless of the situation, several things can cause it:
- Dirty air filter: When your engine has a dirty air filter, it will struggle to get enough air inside to maintain an optimal air-fuel mixture. As a result, the mixture will become rich.
- Faulty sensors: Your engine relies on several sensors to maintain the correct air-fuel ratio. Faulty airflow or oxygen sensors will collect incorrect data, causing the engine to use a rich mixture instead.
- Faulty electronic control unit (ECU): The ECU controls the air-fuel ratio based on sensor data, but if the ECU itself is defective, it could cause a mixture that’s too rich.
There are other reasons for a rich air-fuel mixture, though the ones above are the most common. The important thing to do is troubleshoot your vehicle to rule out all possibilities.
What Causes a Lean Air-Fuel Mixture?
You can also use an air-fuel ratio tester to determine if your car is running on a lean air-fuel mixture. If that happens, it’s likely because of the following:
- A vacuum leak: When a vacuum leak occurs, it allows air to bypass the engine’s usual intake tract, ending up in the fuel and causing too much air.
- Poor fuel pressure: Low fuel pressure can also lead to the same condition simply because there’s too much air in the air-fuel mixture. Again, a mix with excess air is considered ‘lean’.
- Faulty pump regulator: Lastly, a defective pump regulator could also be to blame for your engine's lean air-fuel mixture.
Whether your car is running on a lean or rich air-fuel mixture, you'll need to have a mechanic look at it quickly. The engine will still run, but ignoring the problem will only make things worse.
Check out the Directory at CarpartAU to find the closest workshops to you in your area. Get their contact details to see if they can provide you with the services you require!
By Ray Hasbollah