Basic Motorcycle Fuel Injection System Part 1 (sensors)

Gone are the days of the carburetor in motorcycle. Now, Fuel injection is commonly use motorcycle industry. Because of very strict regulations on emissions nowadays, more and more manufaturer are installing fuel injection system rather than the old school carburetor. Electronic Fuel injections also deliver optimal air-fuel ratio at almost any given riding conditions. It is more economical, less emissions and more efficient.

Basically, The fuel injection system is made up of three main units – the sensors(inputs), the ECU(the brain), and the actuators(outputs). This article will only focus on the sensors – the type of sensors and how it works.

Sensors in the fuel injection system acts like the sense organ(eyes,ears,nose,tongue,skin)  of the human body. They will detect what is happening inside the engine and the tell it to the ECU(The Brains). Just like when you are outside and the rays of the sun radiates to your skin, your skin will act as a sensor and tells your brain that it is hot. The Intake Air Temperature(IAT) Sensor detects the temperature in the intakes, The throttle position sensor(TPS) detects the position of the butterfly valve of the intake manifold, etc. Please note that sensors operate on voltage. Like the Oil temperature sensors, they send voltage to the ECU rather than temperature, The ECU thens converts the voltage to a chart of data to convert it to temperature reading.

Common Sensors use in Motorcycle Fuel Injection

1. Intake Air Temperature Sensor or IATS

2. Intake Air Pressure Sensor or IAPS

3. Throttle Position Sensor (TPS)

4. Engine Oil Temperature Sensor

5. Crankshaft Position Sensor

6. Exygen Sensor or O2 sensor

7. Lean Angle Sensor(to be discussed)


How they Work

1. Intake Air Temperature Sensor or IATS

The intake air temperature sensor detects the temperature at the intake manifold. The Intake air temperature data will be feed to the ECU and the ECU will then calculate Air density based on the temperature.

As Air temperature increases, Air density also decreases. This means that you will be getting less air molecules per engine cycle.

When air temperature decreases(like rainy days), Air density Increases, Meaning you will get more air per engine cycle.

2. Intake Air Pressure Sensor or IAPS

The intake air pressure sensor determines the air pressure in the intake manifold. When riding on the mountains and riding on flat ground air pressure varies greatly. When air pressure varies, so does Air density. Notice, It is harder to breath on mountains than flat ground? This is because on high mountains there is less air, or low air density compared to flat grounds.

When at high grounds, Air pressure detects low pressure. Less Air.

When at low grounds, Air pressure detects high pressure. More Air.

3. Throttle Position Sensor

The throttle position sensor is attached to the butterfly valve to determine how far is the opening of the butterfly valve or if throttle is at full, mid, zero, etc. It acts like a variable resistor wherein at a certain position the resistance of this sensor changes. When electrical resistance varies the output voltage also varies. This variation of voltage will be feed to the ECU.

Image result for throttle position sensor mio

This three sensors IATS, IAPS, TPS sends data to ECU in the from of voltage, Then the ECU converts those voltage to pressure,position or temperature. Then it calculates(based on the values of the three sensors) the Amount of air entering the cylinder for combustion and tells the Fuel injector how much fuel to deliver or how will it stay open. Air Mass flow is CALCULATED and not measured in fuel injection system for motorcycles. This means that changing the different size throttle body, larger injectors, high flow filters etc. will lead to miscalculation of the ECU. All this sensor is located in the Throttle Body.


4. Engine Oil Temperature/ Engine coolant temperature sensor

Engine Temperature sensors detects when engine is cold, when it is on operating temperature or when it is overheating.

When engine is cold this sensor sends voltage to the ECU. The ECU thens tells the fuel injectors to give more fuel. You will notice when you start your engine in the morning, the RPM is high due to more fuel delivered by the injectors.

When Engine is on operating temperature, the ECU tells the fuel injectors to cut additional fuel.

When Engine is overheating, The ECU shuts down. They will only start when engine is back at operating temperature.

This sensor is located in the oil passage or in the radiator

Image result for engine oil temp sensor mio


5.  Crankshaft Position Sensor


The Crankshaft position sensor detects where the crankshaft is in the four stroke cycle.

is it a top dead center?

is it at bottom dead center?

is it at 4 degree before top dead center?

is it time for ignition?

is it time for fuel injection?

Crank Position Sensor

Here are some of the questions that this sensor can answer. This sensor is sometimes called the pulser. This sends signal to the ecu everytime this sensor detects the metal protruding in the flywheel. This will make the ECU determine if it is time for fuel and ignition. This also helps in determining the Air Mass at high speeds.

6. Oxygen Sensor or O2 Sensor

This sensor is located at the exhaust side of the engine. The O2 sensor detects the amount of oxygen present in the exhaust. It will detect if the exhaust burn completely, was it too lean, was it too rich. The O2 sensor then sends data to the ECU and the ECU adjust fuel delivery to reach optimal air-fuel ratio. This is also known as Lamda sensor.

Image result for o2 sensor mio



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