At the 2009 SAE World Congress, there was a sense of urgency in the air as automotive engineers attempt to tackle the U.S. government’s 35-mpg Corporate Average Fuel Economy (CAFE) standard that goes into effect in 2020.
The new standard increases fuel economy by 30 percent from today’s standard. While most manufacturers are taking a multi-faceted approach — which includes diesel, hybrid, fuel cell and electric power sources — one transitional technology is helping to extend the life of the gasoline engine and push fuel mileage and performance forward by leaps and bounds: gasoline direct injection.
The History
Diesel engines always have used direct fuel injection, which squirted fuel, under high pressure, into an engine’s cylinders rather than into the intake manifold. In a diesel engine, the process of injecting fuel directly into the combustion chamber at the top of the compression stroke initiates and controls combustion. The Robert Bosch Company in Germany perfected the modern mechanical injection pump for small diesels in 1927.
Gasoline-powered German and Soviet fighter planes in World War II employed a variant of this direct injection system. Though there were some performance and fuel-economy benefits with direct injection, the most important plus was the ability to provide uninterrupted power during violent dog fighting maneuvers that often caused carbureted engines to sputter and hesitate.
The first automotive direct injection system used to run on gasoline was developed by Bosch, and was introduced on the 1955 Mercedes Benz 300SL. Engine power was double that of its carbureted counterparts and allowed a top speed of up to 161 mph, making it the fastest production car of its time.
Operation
It might be easier to see how direct injection works by comparing it to traditional fuel injection methods first. With a conventional fuel injected engine, all cylinders are supplied with a mist-like mixture of air and fuel at a constant 14.7:1 ratio known as stoichiometric mixture. One or more injector nozzles spray fuel into the air stream being fed to the intake valves. This spray is mixed with air during the intake stroke and flushed into the cylinder, where it is ignited by the spark plug.
The throttle valve determines how much of the air/fuel mixture enters each cylinder. A closed throttle valve means little air in the engine, and thus a small amount of injected fuel, while an open throttle means a lot of air in the engine, demanding a lot of fuel. The air/fuel mixture inside the cylinder can’t deviate very much from the optimum 14.7:1 ratio of air to fuel. In particular, air/fuel mixtures that are too lean simply won’t ignite, creating a excessive NOx and hydrocarbon emission that must then be captured and processed by the vehicle’s catalytic converter.
PAGE 2In a direct injection engine, the injection nozzle is located inside the combustion chamber, rather than in the induction pipe as in multi-port or throttle-body fuel injection. 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 tiny outlet holes creates a well atomized air/fuel mixture.
Each bank of cylinders has a high-pressure fuel rail that feeds the individual injectors and a fuel rail pressure sensor on each rail that helps the vehicle powertrain control module precisely control the fuel pressure. Fuel injectors use internal solenoids to switch on and off the flow of fuel extremely precisely. Fuel flows through six tiny pinholes in each injector.
Injectors are positioned to the side of each cylinder, aiming the fuel directly into the cylinder adjacent to the spark plug and alongside the intake and exhaust valves. Fuel is sprayed into the cylinders at pressures of up to 2,150 psi, about 35 times more intense than port fuel injection.
The spark plug is surrounded by a relatively small, precisely shaped volume of ignitable air/fuel mixture that results when fuel is sprayed toward the spark plug just before ignition. Only the area directly around the spark plug at the top of the cylinder contains air/fuel mixture. Other areas inside the combustion chamber merely contain air or recirculated exhaust gas. This stratification of the charge allows the engine to burn mixtures with a much higher rate of air than conventional engines. Air/fuel ratios can
increase to 60 parts of air (instead of 14.7) for every part of fuel.
As fuel is injected into the cylinder, the shaped piston crown guides the air/fuel mix to the spark plug. As the spark plug fires, igniting the mixture, surrounding areas contain only air or recirculated gases, forming an insulating cushion at the cylinder walls and cylinder head. The cushion of non-combustible gas around the combustion chamber also means that less combustion heat has to be evacuated. This improves the thermal efficiency of the engine, improving fuel economy.
Another factor contributing to improved fuel economy is the ability to increase the compression ratio to nearly 12:1 without the need for premium fuel, because direct injection reduces the tendency of engine knock. The higher compression ratio alone increases efficiency by about two percent.
However, the major fuel reduction potential is realized because of the way we drive. Direct injection charge stratification works best at low and medium loads in the lower half of the engine speed range, where traditional gasoline engines are least efficient. Because most engines operate under these driving conditions, the direct injection engine operates in a stratified-lean mode most of the time, thus increasing fuel economy by nearly 21 percent.
Evolution
The future of direct injection involves coupling the system with other technologies, such as turbocharging and Start/Stop. By playing off the efficiencies of multiple systems, it enables automakers to develop smaller, more fuel-efficient engines while improving torque and performance.
Turbocharging direct injection engines is the most promising fuel economy technology for U.S., according to Paul Whitaker, chief technologist – gasoline engines for AVL Powertrain Engineering Inc., the world’s largest independent, privately owned company for the development of gasoline, diesel and alternative fuel powertrain systems.
“By turbocharging a direct injection engine, it combines existing and proven technologies in a synergistic manner and offers double digit fuel economy benefits with a much lower cost than diesel or hybrid technology,” he says.
It also allows manufacturers to meet future emissions standards using typical catalytic converters and can be applied across a manufacturer’s entire engine portfolio, including Flex Fuel applications.
Ford is working on changes in the coolant system to improve fuel economy for a direct injection equipped vehicles. A typical feature of direct injection engine thermodynamics is the difference in thermal losses, depending on whether the engine is operated in the economy or full-load mode. In the economy mode, an insulating blanket of air and recirculated exhaust gas helps keep heat away from the cylinder walls and head. In the high-powered mode, more heat is released.
A new control system for the coolant circuit is being designed to shut off the fan motor over a longer period of time or reduce the operating speed of the water pump, during economy mode operation, thus reducing operating drag on the engine and improving fuel economy.
GM utilizes Dual Cam Phasing on the camshafts of its Ecotec 2.0-liter Turbo Engine. The phasers continuously vary the intake and exhaust valve timing and use cam position sensors so the engine control module can control the timing accurately. The crankshaft and camshaft position sensors are digital. A new engine controller, specific to the engine, is used to sense and control the engine’s performance parameters.
Variable intake and exhaust timing works synergistically with both the gasoline direct injection and turbocharging systems. The variable engine timing enabled by cam phasing allows the combustion process to be optimized. Also, valve overlap at low rpm can be adjusted by the controller to increase the response of the turbocharger, lessening the feeling of turbo lag.
Servicing
With almost every manufacturer (including Ferrari) having at least one direct injection engine available, technicians should be seeing these vehicles in their bays for service.
“The biggest item to consider when servicing (direct injection) systems is the high voltage and fuel pressures the systems generate,” says Al Krenz, director of service for Bosch North America. A direct injection system typically will operate between 725 psi up to 2050 psi, so bleeding down the fuel system properly is important.
“Always follow the manufactures procedure to bleed the high pressure system down before performing any repairs to the system,” Krenz recommends.
Caution also should be used when diagnosing the voltage signals of the injectors. The high-pressure injectors typically actuate at approximately 70 volts and 10 amps, with the capability to rise over 120 volts.
As with diesel direct injectors, carbon can build up on the tip of the injector and interfere with the distribution and atomization of the fuel. Even the slightest loss of the fuel delivery will have an adverse effect on the engines drivability, power output, fuel economy and exhaust emissions.
Injectors can have varying types of spray patterns, depending on the engine requirements. While typical port injectors produce a fuel droplet of approximately 165 micron, direct injectors atomize a much smaller fuel droplet size of only 65 micron.
The use of direct injection and other complementary systems creates a win-win-win situation for the environmentalists, government and consumers. Environmentalists get a reduction in smog forming emissions, consumers get the high performance they desire from a smaller engine while saving money at the pump and manufacturers are able to achieve better fuel economy, assisting them in reaching the 35.5 mpg national standard. It seems that performance, economy and ecology can peacefully co-exist.