Combustion Chamber Deposits HC

Deposits are formed in the intake system, on the valves, combustion chamber and piston crown after engine operation for several thousand kilometres. The combustion chamber deposits are carbonaceous in composition and porous in nature. Olefins, aromatics and heavier fuels result in higher deposit build up. Engine operation for 50 to 100 hours under cyclic and variable load and speed conditions can result in deposit thickness of around 100 mm in the combustion chamber.
The fuel-air-residual gas charge is compressed into pores of deposits. As the pore size is smaller than the quench distance, the flame cannot penetrate into deposit pores. The unburned mixture comes out of pores during expansion and diffuses back into burned gases. Some of these hydrocarbons will burn up on mixing with the hot burned gases. But, as the temperatures drop on expansion a large fraction of these may fail to get oxidized and are emitted from the engine.
On the other hand, the deposits formed on the combustion chamber surface reduce heat transfer and it may decrease quench layer thickness. Prevention of heat transfer by the combustion chamber deposits increases charge temperatures and hence lower HC emissions. However, the overall effect of deposits is to increase HC emissions.
In the engine and vehicle tests, combustion chamber deposits are seen to increase HC emissions by 10 to 25 percent

Mixture Quality and In-Cylinder Liquid Fuel

Very rich fuel-air mixture has to be supplied during cold starting as fuel evaporation is poor at low engine temperatures. During acceleration, delayed dynamic response of the fuel system to meet the engine requirements again requires supply of overly rich-mixtures. The carburetted engines are to be supplied a richer fuel mixture than the modern PFI engines as there is a delay for the metered fuel in reaching the cylinder. Also, the carburettor is unable to precisely control the fuel quantity.  The port fuel injection systems (PFI) i.e., separate fuel injectors for each cylinder provide more precise fuel metering and more uniform fuel distribution among cylinders. PFI also gives a better control of air-fuel ratio during cold starting and response to transient operation compared to the carburettor.
Fuel injection process in a PFI engine is shown schematically in (Fig 2.13). Mixture preparation is governed by factors such as:

• Fuel atomization and droplet size
• Fuel vaporization on the back of the intake valve depending upon its temperature, and
• Mixing with intake air and hot residual gases. The hot residual gases flow back into the intake manifold as the intake valve opens and its amount depends on the operating conditions. The injected fuel comes into contact with these hot residual gases that help fuel vaporization.

Some features of fuel induction into the cylinder of PFI engines are:

• The conventional PFI system produces the droplets of Sauter mean diameter (SMD) ranging from 130 μm to 300 μm. The droplets larger than 10 μm are unable to follow the air stream and they impinge on the combustion chamber walls producing a non-uniform fuel distribution in the cylinder.
• As the injection is made at the back of intake valve, liquid fuel film is formed at the port and on the back of the valve. The intake air strips this liquid fuel film and carries along into the cylinder. In the process, substantial amount of liquid fuel droplets enter the engine cylinder and is deposited on cylinder walls.
• Shearing of the liquid film from the back of intake valve and port by intake air produces larger droplets than by the injectors which impinge on the cylinder walls depositing liquid fuel film.
• Injection at a higher pressure although would produce finer droplets but the fuel jet velocity and droplet momentum are also higher, which increases the probability of the impingement of the fuel droplets on walls.
•  During cold start as 8 to 15 times of the stoichiometric fuel requirement is injected for the first few cycles, more liquid fuel is deposited inside the cylinder.

The liquid fuel deposition inside the cylinder decreases as the engine is warmed up. During cold starting and warm up, much of the injected fuel remains in the cylinder for several cycles. It vaporizes during and after combustion and thus, contributes to higher HC emissions.  During cold start, with PFI up to 60% higher HC emissions could result compared to fully vaporized and premixed air and fuel mixture. At 90º C coolant temperature, the   contribution of the liquid fuel deposited inside the cylinder to HC emissions is almost zero compared to 20 to 60 percent at 20º C.
In the modern catalyst equipped vehicles, more than 90 percent of HC emissions under standard test driving cycle conditions result during the first minute of operation. due to use of over-rich mixtures during engine start-up and secondly the catalytic converter has not yet warmed up and is not functional.