market dynamics
Why don't motorcycles use in-cylinder direct injection?
Release date:2023.06.15

Why are some high-end motorcycles so expensive?

To answer this question, we first need to understand the advantages and disadvantages of several injection methods.

Advantages of intake manifold injection:

1, the inlet fuel injection whether it is in the air and fuel together into the cylinder, or in the entire compression stroke are constantly rolling and fully mixed.

2, the fuel has more time to absorb heat evaporation, and is evenly mixed with the air, which is conducive to the combustion of all the fuel in the cylinder.

3, the intake valve of the inlet injection is always exposed to the fuel mix, and the gasoline itself is a good solvent, which will reduce the generation of carbon in the back area of the intake valve.

640 (1).png

Disadvantages of intake manifold injection:

1, intake manifold injection because the injection occurs in the intake manifold, some fuel will adhere to the intake pipe along the wall, at the end of the injection, there is a part of the fuel did not have time to enter the combustion chamber, which reduces the injection accuracy and controllability, may have a negative impact on efficiency and emissions.

2, the longer residence time of the air-fuel mixture also leads to the absorption of more heat, and ultimately limits the compression ratio.

Advantages of in-cylinder direct injection system:

1, directly injected into the combustion chamber, the fuel stays for a shorter time after ejection, absorbs less heat, reduces the probability of detonation, is conducive to improving the compression ratio provides more space to increase the compression ratio to improve performance and efficiency, reduce fuel consumption and emissions.

2, the location, because the injection nozzle is located in the combustion chamber, the injection amount is the amount of fuel into the combustion chamber, which improves the injection accuracy and controllability, more conducive to optimizing efficiency and reducing emissions.

Disadvantages of in-cylinder direct injection system:

1, the direct injection in the cylinder does not have enough time to vaporize the fuel, it only begins to inject oil in the second half of the compression stroke, can only be made up by having an increase in fuel pressure, in addition, the use of a special cavity structure piston, forcing the production of air fuel mixture tumbling and spinning, but nonetheless, at low engine speed, Direct injection can also suffer from an uneven mix of air and fuel. Since the piston speed is lower, which means that the piston is pushing the mixture more slowly, resulting in less tumbling spin, and therefore a higher percentage of incomplete combustion will occur, reducing efficiency and increasing emissions. Even at high RPM, direct injection in the cylinder will not provide enough fuel, assuming the engine is running at 6500 RPM. The engine takes only 9 milliseconds per rotation, which means that each stroke takes only 4.5 milliseconds, but the direct injection in the cylinder setting, only in the second half of the compression stroke start injection, only about 2.25 milliseconds to fuel injection because the injection time is very short, as the speed increases, neither the injector capacity nor the oil pressure can meet the demand. This is why the average RPM of an in-cylinder direct injection engine is limited to about 6,500 RPM.

2, on the contrary, direct injection in the cylinder can never be injected on the back of the valve, which will lead to the accumulation of carbon and other waste from the PVC system over time, reducing performance and leading to unstable operation. Although preventive measures such as fuel cleaners or valve cleaning can be used, they also increase the maintenance cost of in-cylinder direct injection engines. The carbon buildup behind the valve can also lead to low-speed pre-ignition of the LSPI, which occurs at low speeds and high loads, that is, at full throttle, once particles or oil droplets enter the combustion chamber and, together with unmixed fuel, are subsequently exposed to the extremely high compression inside the engine to produce detonation. If the particles continue to exist after the explosion, it will lead to catastrophic damage such as cylinder pulling.

In addition, there are also engines with injectors inside and outside the cylinder, which is the so-called dual injection system, this way can indeed learn from each other and achieve the ideal state as much as possible, but the cost is to increase the number of moving parts and manufacturing costs.

(Graphic from the network, if there is infringement, please contact delete)