THE ECOBOOST ENGINE: COMBINING VARIABLE VALVE TIMING ...

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Neil Debski Seth Kahanov However, before direct fuel application was emphasized, engine design went through a period of inefficient processes and performances. THE INEFFICIENT WORLD OF ENGINES BEFORE THE ECOBOOST Before the technology of the EcoBoost engine was introduced to the automotive market by Ford, engine efficiency always compromised either the fuel economy or fuel efficiency of the engine. The internal combustion of conventional engines consisted of a less controlled fuel application to the engine. This resulted in a greater demand of fuel for the engine since most of the fuel was being wasted and not applied to crucial parts of the engine. Every engine design is based off of a translation of the standard Otto cycle, as seen in Figure 1. FIGURE 1 Process of the Otto Cycle [6] The Otto cycle begins after the engine has taken in both fuel and air and is then adiabatically compressed (without any heat gain or loss [7] [8]. After this compression, the temperature and pressure are increased at a constant volume [8]. The piston with which the compression was applied is then quickly removed from the resources, which causes adiabatic expansion. At this point, the exhaust valve within the engine is opened, which reduces the pressure of the system back to atmospheric pressure [8]. The piston then eliminates the exhaust gasses in the system in order for the Otto cycle to complete and begin again [8]. Steps within this cycle have become very harmful when dealt with in an unbalanced manner. When the compression of the air and fuel is not done within a precise area, a large amount of fuel is spread out within the engine and not compressed with the air. This fuel is eventually not included in the engine’s usable energy and is then swept out of the engine in a later step of the Otto cycle with other exhaust gasses. What was once fuel with the potential to better the function of a car is now a harmful pollutant that only serves to worsen the environment in which it is released. This unbalance of concerns regarding fuel injection and fuel application has been a downfall of several engines of the past, including the majority of conventional four-stroke engines. A major disadvantage in standard four-stroke cylinders has been “the inability to over-rev [perform despite demands from the driver that exceed the engine’s abilities]. A low cost poppet valve train typically [lost] control around 12000 rpm or less” [9]. This results in drivers not being able to control the type of engine performance they need for different types of travel they could need at any time. Another drawback from previous two-stroke and four-stroke engines is that they were “unsuitable for heavy fuel operation due to difficulties in starting, low power, detonation sensitivity and carbonization” [9]. These flaws in engine efficiency have led to high levels of fuel waste. This waste of gasoline makes the driving experience increasingly expensive. Recent studies have concluded that more emphasis needs to be put on the fuel injection design. Early improvements to more precise injection, done in the MinDwell design, the first engine design which focused on a new, direct fuel injection, proved that “application of the MinDwell injection strategies yielded a reduction in emissions, fuel consumption and combustion noise” [9] This application of injection precision led to the engine running “extremely well on heavy fuels . . . which is a unique capability for an Otto cycle spark ignition engine without resorting to high pressure injection technology” [9]. Since the Otto cycle leads to flaws when one step is done inefficiently, the entire cycle becomes more productive when the first step is handled with more precise fuel injection. This improvement of the engine’s Otto cycle allowed the engine to be more beneficial to both the driver and the environment. This realization was displayed in Ford’s design of the EcoBoost engine, which Ford chairman John Fleming referred to as “a key element of Ford Motor Company’s global blueprint for sustainability” [10]. THE NECESSITY OF DIRECT INJECTION Direct injection engines are becoming more common in automobile manufacturing to improve fuel efficiency and horsepower in engines. Ford Motor Company uses spark ignited direct injection (SIDI) technology in its EcoBoost engine. SIDI engines inject gasoline directly into the air in the cylinder where the resulting mixture is then ignited by a spark plug [11]. In SIDI, the fuel is injected at a high pressure of 2,176 psi as it is injected into the combustion chamber and not the manifold like in port injection. Benefits of SIDI include lower emissions especially at startup, higher compression, better fuel economy on turbocharged engines, and increased horsepower when compared to port injection in Figure 2. [11]. 2
Neil Debski Seth Kahanov FIGURE 2 Port Injection vs. Direct Injection [12] All gasoline engines depend on atomizing the fuel before combustion. Unfortunately, with carburetors, throttle bodies, and even port injection, the atomization takes place at a distance from the combustion chamber. By the time the atomized fuel and air reach the combustion chamber, some of the fuel has separated out and collects on intake. However, when fuel atomizes and the air and fuel mixture are cooled in the combustion chamber, a higher compression ratio is produced thus eliminating the problem. Better atomization contributes to a reduction in pre-ignition and detonation, which is why the SIDI engine can operate at a higher compression ratio and consume less fuel [12]. SIDI allows the engine to run on a leaner mixture (more air, less fuel) which allows for better fuel economy at part and full throttle [12]. The fuel delivery in a SIDI engine comprises of two components. The low side works with a fuel pump at 60 psi to deliver fuel from the tank to the engine compartment. The high side, which is located by the cylinder head, is driven by lobes on the camshaft and delivers fuel at 2,176 psi [12]. The high side, high pressure pump utilizes the fuel rail’s pressure with an engine control module. This allows more accurate control over fuel metering (the amount of fuel injected), and injection timing (exactly when the fuel is introduced into the cylinder). Engine knocking is the premature fuel combustion that can result in power loss of the engine. Engine knocking is compression detonation or pre-ignition of fuel in the power stroke of the engine. Knock is a function of heat and pressure. The fix for engine knock until now has been to reduce the compression ratio of turbocharged engines. The compression ratio is the ratio between the cylinder's maximum and minimum volumes, when the piston is at the bottom of its stroke versus the top [13]. With direct injection, the spray of fuel during the compression stroke helps keep the cylinder cooler, reducing the possibility of knocking [13]. Like the spray from an atomizer bottle one might use to keep cool in the summer, the fine mist generated by each solenoid-controlled injector’s six tiny outlet holes helps to create a well-mixed air-fuel mixture. It also cools the incoming air, helping to reduce the potential for engine knock. This allows for higher compression ratios which can boost fuel efficiency by a few percent. When coupled with sophisticated electronic control of how much fuel to add with the turbo, the end result can be up to a 20 percent increase in fuel efficiency [11]. Better atomization contributes to a reduction in pre-ignition and detonation, which is why the spark ignition direct injection engine can operate at a higher compression ratio and consume less fuel. The EcoBoost engine has both more peak torque and lower end torque, meaning direct injection produces a remarkable flat torque curve. The EcoBoost engine produces 90 percent of its peak torque between 1,550 and 5,500 rpm. About 98 percent of all driving is between 1,000 and 3,000 rpm [14]. THE CRUCIAL TIMING OF VARIABLE VALVE ACTIVITY The next aspect of the EcoBoost engine that is crucial to its design is its variable valve timing. This section will explain how the EcoBoost’s use of variable valve timing makes the engine fuel efficient as it works along with the direct fuel injection. We will define variable valve timing by describing how less overlap between intake and exhaust strokes allows for better efficiency at low and high speeds alike. Valve overlap is the time when both intake and exhaust valves are open, giving an opportunity for exhaust gas flow and intake flow to influence each other. For maximum efficiency, valve overlap should allow exiting exhaust gas to draw an intake charge into the cylinder without intake gas passing straight to the exhaust system [15]. At high speeds and torque, output is improved due to the aided intake of fresh charge due to increased valve timing. At low speed, the effect of valve overlap is to reintroduce exhaust gasses into the combustion chamber [15]. This is known as generating internal exhaust gas recirculation or internal EGR. Exhaust gas recirculation also reduces hydrocarbon and NOX emissions by the recirculation of un-burnt exhaust gasses [15]. The retained exhaust gasses tend to be very rich in un-burnt hydrocarbons, as they typically come from the end of the exhaust stroke. This strategy can therefore be quite effective at reducing emissions. Each of the three implementations of variable valve timing uses a cam phaser to adjust the camshaft angular position relative to the crankshaft position, referred to as camshaft phasing [15]. A camshaft is a rotating cylindrical shaft used to regulate the injection of vaporized fuel in an internal combustion engine, as seen in Figure 3. The crankshaft is located in the engine of a vehicle and converts the force created by the engine's pistons moving up and down into a force that moves the wheels in a circular motion so the car can go forward. 3
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