Air and Fuel Induction Lecture 3 1
Outline In this lecture we will discuss the following: A/F mixture preparation in gasoline engines using carburetion. Air Charging technologies: Superchargers Turbochargers 2
As discussed in the previous lecture, there are two main methods for A/F mixture preparation in gasoline engines: Fuel injection Carburetion Until the 1980s, carburetion used to be the universal technology adopted by all car manufacturers. Despite the many shortcomings of carburettors, they are relatively simple and inexpensive. 3
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A carburettor system basically consists of: Venturi tube Throttle plate Fuel capillary tube Fuel reservoir Choke 5
Air enters the carburettor at nearly atmospheric pressure, and flows in the direction of the lower pressure in the engine cylinders. Due to the losses the flow encounters as it moves through the system, a pressure differential is created across the fuel capillary tube. This forces the fuel in the capillary tube to flow into the venturi throat in the form of droplets that evaporate and mix with the air in the intake manifold. 6
The purpose of the venturi tube is to: increase the pressure drop and hence the pressure differential. increase the velocity of the air. Note that the pressure drop in the venturi tube depends on the velocity of the flow: P V 2 As a result, as the air flow rate increases, the pressure differential increases. This increases the fuel flow and thus the A/F ratio can (roughly) be maintained at the desired level. 7
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It must be noted that there are two factors that act against each other with regard to the sizing of the venturi. Increasing the diameter will decrease the pressure drop and thus decreases the fuel flow and the fuel control. This however, will increase the volumetric efficiency, and vice-versa. One way to overcome this dilemma, is by using a primary and a secondary venturi. 9
The secondary venturi is used to give good control of the fuel while the larger one ensures minimum losses regarding the volumetric efficiency. 10
The speed of the engine (power demand) is controlled by the throttle which determines the air flow rate. At high speed or acceleration operation, the throttle is wide-open (WOT) which is also characterised by a rich mixture. At steady state cruising conditions, the engine operates near stoichiometry or slightly lean conditions. At idle conditions, the throttle is nearly closed, and thus minimising the flow rate through the venturi. This leads to only a minimal pressure drop and very low fuel flow rate and consequently to poor fuel flow control. To avoid the possible resulting misfires, an idle valve is added downstream of the venturi with the aim of increasing the fuel concentration. 11
The choke is used to overcome the cold start difficulties. When the choke valve is closed, it restricts the air flow and creates a vacuum in the entire intake system. This results in a large pressure differential across the fuel capillary tube and the idle valve, which leads to a large fuel flow. This gives a very rich mixture (very low A/F ratios). 12
But why does this need to be done at cold conditions? Cold fuel: Higher viscosity, slower flow, larger droplets Cold engine parts This leads to much less effective vaporisation of the fuel even if the fuel liquid flow is sufficient. This is made even worse with the fact that the flow is very low since the engine starts at a low speed. 13
The disadvantages of a carburettor system can be summarised as follows: Bad fuel economy Slow and difficult control of the fuel flow and mixture strength Large pumping losses Difficulty in ensuring homogeneity of the mixture Icing at the throttle due to evaporative and expansion cooling 14
Charging Systems The idea behind charging is increasing the amount of the mixture entering the cylinder through increasing the pressure of the intake charge. This results in an increased power output. Charging can be done either through superchargers or turbochargers. This is applied now to all two-stroke engines, most C.I. engines and many S.I. engines. 15
Superchargers Vs Turbochargers Superchargers are driven directly off the crankshaft. Additional parasitic load on the engine. Higher cost and noise. Much quicker response. Turbochargers are driven by a turbine utilising the exhaust gases. No parasitic loads. Suffers from turbolag which occurs during sudden throttle change. 16
Turbochargers Many modern turbochargers are variable geometry turbochargers. The turbine blade angles change according to the flow rate to give maximum efficiency. 17
Charge inter-cooling It must be noted that charging not only increases the intake pressure, but also the intake temperature. This is undesirable in S.I. engines, since it leads to decreasing the ignition delay and thus to knocking. In C.I. engines, this can lead to increased NOx emissions. As a result, most superchargers/turbochargers are equipped with an intercooler to decrease the charge temperature. 18
Charge inter-cooling 19
Supercharger Efficiency The isentropic efficiency of a supercharger can be given as follows: η sc = w isen /w act η sc = (T 2s -T 1 )/(T 2a -T 1 ) Where, w isen : Isentropic work w act : actual work T : Temperature T 2s / T 1 = (P 2 /P 1 ) (γ-1)/γ 20
Turbocharger Efficiency η turbo = η s turb η s comp η m Where, η s comp = (T 2s -T 1 )/(T 2a -T 1 ) η s turb = (T 1 -T 2a )/(T 1 -T 2s ) η m = w ca / w t a Typical values are in the range of 80-90% 21