Mastering Fuel Injection
How Mercedes-Benz pioneered this key technology
Article Karl Ludvigsen
images Karl Ludvigsen, Daimler Archives
As part of its traditional and close co-operation with Bosch, Daimler-Benz had been working since the mid-1920s researching diesel engines for heavy vehicles. The Bosch role was to develop plunger pumps that delivered diesel oil directly to the combustion chambers of test engines, their exact fuel dosages timed precisely to each cylinder’s needs.
The next step, reasoned Stuttgart engineers, was to use similar technology to deliver fuel to a new generation of gasoline-burning aviation engines based on the spark-ignition Otto cycle. That this could be achieved was by no means certain.
An unprecedented challenge
“It is surely remarkable,” wrote aircraft expert Bill Gunston, “that both Daimler-Benz and Junkers chose in 1933 to develop direct fuel injection for their next generation of high-power spark-ignition aircraft engines.” No engine manufacturer had ever done this before. Although a few early engines, including those of Orville and Wilbur Wright, had a form of direct delivery of fuel to their ports, only diesel designers had ever tried to pump fuel directly into cylinders.
The task was daunting, according to Gunston. “To squirt a microscopic dose of fuel into each cylinder in turn, so that the engine not only runs properly but also achieves minimum fuel consumption, is a severe challenge. This is partly because the injection pressure has to be about 500 pounds per square inch and partly because the amount of fuel injected in each dose is almost vanishingly small: For a 240-bhp engine, about 0.0002 pound or 0.0077 cubic inch, smaller than a grain of sugar.
To achieve this, Gunston said, “Manufacturing tolerances have to be in the order of five-millionths of an inch, never previously called for in any branch of engineering.” As well, gasoline was anything but a good lubricant, unlike the diesel oil that Bosch successfully pumped into diesel engines. In addition, spark-ignition engines had to operate perfectly over a much greater speed range than truck and stationary diesels.
Important advantages
Despite these challenges, Daimler-Benz engineers chose to tackle direct injection for the new V-12 aero engine the firm was designing in the early 1930s: They foresaw major advantages. One was lower fuel consumption thanks to injection toward a spark plug, in an area of rich mixture, with a second plug near the injector igniting a leaner mixture, thus stratifying ignition of the air-fuel mixture. Fuel economy also benefited from identical distribution to all 12 cylinders, none getting an increase to compensate for uneven distribution among cylinders.
Testing revealed the capacity to run higher cylinder pressures – from supercharging or a high-compression ratio or both – without suffering from damaging preignition or knock. Working on the engine were Fritz Nallinger and Hans Scherenberg; their 1937 patent stated: “Apart from the lower tendency to knock, such an arrangement has the advantage of higher performance with relatively low fuel consumption compared with that with only one or more spark plugs opposite one another and located in a zone of richer mixture.”
An overwhelming advantage of injection for an engine powering a dynamic vehicle was total immunity from the forces of gravity acting on the fuel system. Here, carburetors were notoriously fallible, their float chambers succumbing to g-forces that delivered either too much fuel or too little, in both cases stalling engines in the midst of aerobatics or hard cornering. This was an advantage worth hard work to achieve.
Taking flight
The first Untertürkheim trials were made with the single-cylinder 2,820cc test engine built for the DB 600, as the new V-12 aero engine was known. Both the DB 600 and the Junkers Jumo 211 V-12 were inverted vees with their crankshafts at the top to give the pilot a better view. Through exacting tests, the many advantages mentioned above were verified. Both engines had Bosch 12-cylinder injection pumps, an inline unit for the Mercedes and a flat-12 version for the Junkers.
“Each pump group was made like a fine watch,” Gunston said, “with complex anti-backlash linkages giving a precise stroke to the tiny plungers feeding the cylinders, the original input timing being by a camshaft. Inputs varying the stroke were provided by sensors measuring altitude and boost pressure to give fuel delivery proportional to charge density. The measured doses were delivered to nozzles flush with the cylinder wall with three radial spray holes.”
While the DB 600 was carbureted, the definitive DB 601 of 1935 had fuel injection, as did all its successors during the war. “In the next few years, output increased by leaps and bounds,” according to Scherenberg, “This was achieved by increasing supercharger boost while simultaneously injecting methanol in front of the supercharger.” The first full-production version was the DB 601A, which produced 1,175 horsepower at 2,500 rpm at takeoff from its 35.7 liters. It was the mainstay of many of Germany’s best fighters and light bombers.
Obviously, the Mercedes-Benz racing engineers couldn’t overlook the advantages of fuel injection demonstrated in the aero engines. Starting in 1937, researcher Herbert Maruhn commenced trials of direct injection on a single-cylinder 420cc test built in 1933 to test the basic engineering and potential of the 1934 Grand Prix straight eight. His aim was to evaluate different injection-nozzle designs and placements to use in the 1938 3.0-liter V-12 racing engine.
In June 1937, engineering chief Max Sailer stated that the new V-12 would be designed as an injected engine, but would also have provisions for carburetion. However, Maruhn’s antiquated test unit gave such random results that fuel injection was set aside to await the needs of unsupercharged Mercedes-Benz racing engines 14 years in the future.
Early postwar development
Work began at a more modest speed after the war with trials of injection from 1947 through April 1949 on the 4-cylinder engine of the 170V sedan. This research, conducted by Heinz Hoffmann alongside his work on diesels, used modified aircraft-engine injection nozzles. He was able to achieve a 23.6-percent power improvement with no deterioration in brake-specific fuel consumption.
During the war years, Scherenberg had been deeply involved in the development of the many variants of petrol-injected Daimler-Benz aircraft engines. Assisting him in this since May 1939 was Karl-Heinz Göschel. In the later 1940s, Scherenberg became chief engineer of the small car-manufacturing firm of Gutbrod. There, with the help of Bosch and his colleague Göschel, he pioneered the production application of gasoline injection to the two-stroke engine of the Gutbrod car. Both men returned to Daimler-Benz in 1952.
The 300SL and 300S
Thus, an unmatched repository of fuel-injection know-how awaited its application to the 300SL engine under the overall technical direction of Nallinger. With his colleague Hans Grötzinger, Göschel made injection-engine development his priority for 1952. They carried out a multitude of trials that yielded excellent results by the end of the year. How did they achieve so much so quickly? “We worked hard,” Göschel said, “and had no time for meetings.”
They faced tough challenges in moving from the injection requirements of huge aircraft-engine cylinders to the much smaller auto engine units, as Maruhn had already found. They also had to cope with the varying load and speed conditions experienced in cars. Would injection be better into the inlet manifold or directly into the combustion chamber? Both versions were tried on a pair of test engines.
Direct injection showed itself the most promising. One of Göschel’s 3.0-liter sixes developed a highly encouraging 188 brake horsepower at a moderate 5,300 rpm and peak torque of 201 pound-feet at 4,300. Spark plugs remained in the cylinder head while the former plug positions in the block, the ones used by the original 300 six, were taken up by the injection nozzles. There they sprayed from the hot to the cool side of the chamber – across the faces of the valves – during the intake stroke.
By December 1952, direct injection gave Göschel’s experimental 6-cylinder engine 214 horsepower at 5,960 rpm and 211 pound-feet of torque at 4,760 rpm. This became the target figure for the M198 engines to be built for the 1953 competition 300SL. Although that car never raced, it proved the merits of injection so effectively that it was adopted for the production 300SL introduced in 1954.
As used in the 300SL, the 3.0-liter six developed 215 horsepower. At the Frankfurt Auto Show in May 1955, direct fuel injection also made its bow in the elegant 300S. Although this version was officially the 300Sc, most catalogs continued to use the simpler 300S designation. Its M199 engine was rated at 175 brake horsepower at 5,400 rpm.
The W196 Grand Prix engine
Parallel to the launch of its glorious 300SL, Mercedes-Benz was also committing its W196 to the Grand Prix wars and, in 1955, its 300SLR to sports-car competition. The positive results obtained in the 1952 direct-injection tests with the 300SL engine assured that the Grand Prix engine would be designed from scratch to be injected. As with the 300SL, Göschel led the experimental work in cooperation with system supplier Bosch.
Different injection-nozzle positions in the inlet port, as well as in the cylinder, were tested. A point on the cylinder wall just below the inlet valve was selected, with the centerline of the nozzle angled upward at 12.5 degrees. Its spray emerged in a 30-degree cone at a pressure of some 80 bar. Flow began 30 degrees after top dead center on the inlet stroke and continued for 160 degrees.
This painstaking experimental work brought the output of the Formula One M196 to 290 horsepower at 8,500 rpm by the end of 1955. As the highest power available to Grand Prix drivers in the 1954-55 period, this was in fact equaled by only a handful of competitors during the rest of the 2.5-liter formula period through 1960.
Mechanical port injection
Production of the last 300SL roadsters in 1963 marked the end of direct fuel injection – for the time being. Still, injection continued in a different format, with fuel being introduced into the inlet ports instead of directly into the cylinders. First introduced in the 300d, the system used two plungers to supply the cylinders in groups of three.
Although this departed from the strictly timed direct injection, it worked well enough.
In late 1958, Mercedes-Benz introduced this mechanical port-injection system in large-scale production for the first time. The Mercedes-Benz 220SE, the last evolution of the Ponton 220 series and launched in 1954, had this manifold injection. It gave 15 horsepower more than the carbureted engine and much quicker dynamic response, plus slightly improved fuel economy. From that time, the letter “E” – for Einspritzung – in the model designation became a hallmark of injected performance in Mercedes-Benz models. Through 1972, this in-house system was used in the company’s models where appropriate.
Accelerated development
Work to refine fuel injection accelerated in the early 1960s in response to the Clean Air Act in the United States. At first, a simplified Bosch mechanical system was adopted, first for the 1963 230SL. Meanwhile, in cooperation with America’s Bendix, Bosch developed an electronically controlled system it called “Jetronic.” The first Mercedes-Benz to use it was the 1968 250CE coupe, followed in 1969 by the V-8 in the 350.
With many carmakers and Bosch insiders still preferring mechanical injection, in 1973 the company divided its offerings between the electronically controlled D-Jetronic – now used by Mercedes – and mechanical L-Jetronic, a constant-flow mechanical system. While the former was used in models for sale in markets with tough emissions controls, many owners found the latter better suited to high performance and racing.
In 1979 Bosch unveiled its Motronic injection system, which achieved better emissions suppression by controlling both ignition and fuel delivery. This soon made its way into racing as well and was used by the Sauber-Mercedes Group C cars of the 1980s.
Direct injection today
The wheel turned full circle in the 21st century with the introduction of direct fuel injection at the end of 2002 in a new 1.8-liter, 4-cylinder engine. At last the technology existed to implement direct injection in the manner planned half a century earlier by Nallinger and Scherenberg, whose holy grail was the use of injection to create a stratification of the fresh charge that would improve ignition and reduce fuel consumption. Although that was their aim, the technology of their day didn’t allow its full implementation.
Research work in the early 1990s contributed to the breakthrough. Stratifying the charge required a uniform spray of fuel that was stable under all operating conditions in the immediate area of the spark plugs. Called a “spray-guided combustion system,” this was much more technically demanding than the previous “wall-guided” process in which mixture formation chiefly depended on the motion of the charge in the cylinder, which was not consistent.
The piezo puzzle
The goal of creating a spray of fuel that was always uniform and precise required the development of a completely new injector, leading engineers to piezoelectric technology. This uses special ceramics and metal alloys that change their shape within milliseconds when subjected to an electrical impulse. In the automotive world, the term "piezo" has only been in general use since 2004, when the first diesel engines with third-generation common-rail injection entered the market.
The piezo module in the latest family of Mercedes-Benz gasoline engines directly controls the injector needle. Piezo motion is translated into needle movement, determining the amount of flow through the valve. Thanks to its consistent stroke, piezo technology ensures a reproducible spray pattern to achieve effective control of the combustion process.
As important as consistency and controllability was the shape of the injected spray of fuel. A new type of injector opened outward to create an annular gap just a few microns wide. The shapes of the gap and the nozzle form the spray pattern. Under all injection and operating conditions, the result is a uniform, hollow-cone-shaped spray that retains its shape when the engine-management system changes the timing of the intake camshafts or the length of the intake manifold when high output is demanded. High fuel pressure of 200 bar – more than double the pressure used in the 1950s – also makes a major contribution to the fuel nozzle’s pattern stability.
The piezo injector of the CGI (charged gasoline injection) engine extends down into the center of the 4-valve combustion chamber. There it displaces the spark plug, which was relocated nearer the exhaust valves so it can reach the ignitable mixture at the turbulent edges of the cone-shaped spray.
So fast is the operation of piezoelectric injectors that they can pump several successive jets of fuel into the combustion chambers during each working stroke. This makes combustion more rapid, uniform and complete than a single-injection system, delivering improved thermodynamic efficiency. Moreover, the engine’s emission of hydrocarbons is reduced by more than half.
Complementing and controlling spray-guided combustion is a new piston design in which the crown is channeled to concentrate a lean mixture in the area around the spark plug while preventing it from spreading out toward the cylinder wall. Cross-flow cooling of the cylinder head helps keep spark plugs and injectors at their most advantageous temperature range.
The end result of the CGI system is an engine that significantly reduces fuel consumption and pollutant emissions compared with a similar engine with manifold fuel injection, while at the same time providing higher power and torque. If that sounds like an ideal fuel-delivery system, it’s the ultimate reward for the research process that the Untertürkheim engineers began 69 years before their 2002 introduction of a system that is keeping the 142-year-old spark-ignition Otto engine well in the race for survival against heavy competition.
1. Although Mercedes-Benz began work on fuel injection in the 1930s, its practical application was limited to aircraft until fuel injection was added to the 1952 300SL for the 1953 racing season. Hand-fabricated ram pipes were fed air by a plenum chamber and forward throttle.
2. An inverted V-12 aero engine, the Mercedes-Benz DB 601 was a mainstay of the German Luftwaffe during WWII. Its direct fuel injection offered an important advantage in dogfights.
3. The Messerschmitt Me 109 fighter was one of the Luftwaffe aircraft powered by the DB 601.
4. As shown in Max Millar’s cutaway, the Daimler-Benz DB 601’s injection pump was in its central vee and driven from the gear train at the rear of the engine. The pump had 12 individual injection pumps in line; its advanced fuel-control system could compensate for both altitude and demand.
5. An early drawing of the M154 engine of the 1938 Mercedes-Benz 3.0-liter V-12 showed a Bosch 6-cylinder injection pump along each side of the crankcase. The bosses for the injection nozzles are just visible below the exhaust ports.
6. In the early 1950s, Hans Scherenberg successfully fitted direct fuel injection to a Gutbrod two-stroke engine. His patent showed the position of the injection nozzle at (2).
7. In 1953, Scherenberg patented the positioning of the injection nozzle of the 3.0-liter Mercedes six using the opening in the block formerly used by the spark plug.
8. The sole prototype 1953 300SL racer. Lighter than the 1952 model, it had fuel injection, bigger wheels, better brakes and a rear-mounted gearbox. Daimler-Benz chose not to race it, concentrating instead on the W196 Grand Prix car.
9. The first production car with direct fuel injection, the 1954 300SL, used a pump and equal-length fuel pipes to the nozzles, shown here.
10. This cutaway of a 1954 300SL illustrates the inclined straight-6, with its induction ram pipes, injection-nozzle positions and spark plugs now located in the cylinder head.
11. A prototype engine for the 300SL reveals the shaft drive for its injection pump running from the forward accessory drivetrain (arrow). An oil pump opposite the drive to the injection pump serviced its dry-sump system.
12. This under-hood view of a production 300SL roadster showcases the powerful machine’s distinctive cast-aluminum inlet ram pipes designed for fuel injection.
13. A detailed look at a 1955 300SLR racecar’s direct-injection system reveals the beautifully curved tubing routed to the respective cylinders. Just as in the 300SL production sports car, an electric pump purged the system of residual vapor.
14. A cross-section drawing of the 1955 300SLR racecar’s 8-cylinder 3.0-liter engine shows the fuel-injection pump at upper right and its pipe to the nozzle in the cylinder wall.
15. The benefits of fuel injection were brought to large-scale production in the 1958 Mercedes-Benz 220SE 6-cylinder model; the system used two pumps, each of which supplied three cylinders.
16. The M110 engine of the Mercedes-Benz 280E had Bosch D-Jetronic with a plenum chamber for air supply and vee-inclined overhead valves.
17. Under the hood of the 280E and 280CE models was the M110 straight six with aluminum cylinder head and D-Jetronic fuel injection.
18. In 2002, Mercedes-Benz returned to designing engines with direct fuel injection; here, the fuel nozzle is positioned vertically above the shaped piston crown and the spark plug offset to the side.
19. As fitted to a contemporary Mercedes-Benz V-6 engine, the direct-injection system has a high-pressure pump and cooler to suppress vapor lock. Each cylinder has its own piezo injector.
20. With its twin overhead cams, 4-valve heads and direct fuel injection, today’s Mercedes-Benz V-6 is well equipped to cope with future performance demands and emissions requirements.