EFI and the Marvels of Mechanical Fuel Injection

The single biggest achievement of automobile improvement in the last four centuries is Electronic Fuel Injection (EFI). Not one single achievement has made as many technical advancements in engine building design than the ability to accurately supply and monitor fuel combustion. Carburetors worked well when re-jetted correctly but did not offer much versatility for us mountain folk. EFI improved fuel consumption by implementing a closed loop (oxygen sensor that sends input to the computer that the combustion is either too rich or too lean) system to adjust the amount of fuel each injection pumped into the engine.

This also allowed for more complex systems to be fitted easily. Turbochargers, different cams, or modifications could be made and with a proper tune, the engine could adjust to conditions without a problem. Suddenly it was possible to make power up high because the EFI could keep up with the demands of a turbo. Mechanical fuel injection is a work of art, don’t get me wrong. The Kugelfisher system powered the BMW M1 to victory, the 2002 tii to cult car status, and the 2002 turbo past unimaginable barriers. It is a very impressive feat of engineering. However the one problem with the Kugelfischer was that is could not be changed to support more fuel. This meant basically any modification on these cars would not have the fuel system to support it.

The interior cone of the kugelfischer has a 3D profile, much like a cam, but instead of in one dimension, in three dimensions with inputs being engine speed, throttle position, and the profile of the cone guide a roller bearing which adjusts the fuel input. Instead of engine speed, the 2002 turbo uses inlet manifold pressure. As these two inputs adjust the cone in the X and Z axis, the profile of the cam was designed by German skunkworks type engineers to match the exact fuel requirements of the engine and moves the roller in the Y axis. Thus a large change in the fuel requirements leads to a custom cone in the mechanical fuel unit.

Only the 2002tii and 2002 Turbo cones were made (I’m not sure anybody with an M1 would volunteer to take their fuel system apart to compare, would they?) and aside from 3D modelling and something like a 3D printer would it be possible to even try to create a custom map. Needless to say, the mechanical fuel injection system was archaic. (If you want to learn more about it, check out this neat article)

Below is an article posted on the M5 Car Blog about electronic fuel delivery in the F10 M5 and some basics on how Electronic Fuel Injection works.

An innovation in the new BMW engines, including the S63top in the F10 M5, is “Gasoline Direct Injection” (GDI). In GDI, fuel is injected directly into the cylinder.

The F10 M5 has very precise control of fuel delivery directly into the cylinder. This results in quicker throttle response and more efficient use of fuel with reduced emissions. Diesels use direct fuel injection into the cylinders, but this has not come to gasoline powered cars until recently.

Fuel injection was introduced in cars in the 1950’s but was not in widespread use until emissions controls forced manufacturers to move to fuel injection in the 1980’s. Previous to this, cars had what was knows as a carburetor.

A carb is a relatively simple device in theory, but more complex in practice. The theory is that the intake air blows past a fuel nozzle and the pressure of the airflow sucks the fuel out in droplets. To make the air stream move faster past the nozzle, a so-called Venturi tube which temporarily narrows the air passage is used.

The complexities arise in controlling the flow of fuel to be just right for all situations. In those pre-computer days this had to be accomplished by purely mechanical means which is what made actual carburetors complex and finicky.

The first fuel injector systems were called “single point” injection, and used a mechanical pump to continuously inject the fuel through a small hole in the fuel nozzle into the throttle body. The relatively high pump pressure and the small nozzle more effectively vapourized the fuel. Think of squirting perfume out of a perfume bottle under pressure – the fine mist generated is similar in principle.

These systems used mechanical means to control how much fuel was injected, though some later innovations used electronics to control the fuel pump and thus the amount of fuel.

A further innovation was to move entirely to a new digital model of electronic control, in which the pump pressure was maintained at a constant value, but electro-mechanical devices (Solenoid Fuel Injectors) pulsed the fuel into the airflow. The more frequent the pulses and the longer the pulse width, the more fuel was delivered. These systems were knows as Electronic Fuel Injection (EFI).

The earliest EFI system was from the US Bendix Corporation (since acquired by Honewell) which first introduced the concept in 1959 in the Electrojector system in the 1958 Chrysler 300C and some other cars.

The system used discrete transistors which proved to be unreliable and so it was discontinued soon after its introduction. Bendix sold their patents to the German company Robert Bosch GmbH who named their EFI system “Jetronic” and had a number of different variants from 1967 until 1995. Some of these were mechanical systems (K-Jetronics), others used analog electronics, but most used digital electronics to control the fuel pressure pump. Some were single point fuel injection, and others were multi-port fuel injection, where fuel was injected just upstream of each cylinder’s intake valves.

An EFI system uses a fuel injector which contains a “solenoid”, which is a winding of wires around an iron core. When electricity is put through the wound wires it generates a magnetic field that acts to move a plunger and open a small valve at the spray tip. Pressurized fuel behind the tip is then vaporized out the tiny nozzle into the airflow.

The Jetronics system was superseded by Bosch’s Motronic system, whose first ever use was by BMW in the E23 732i in 1979

Motronic integrated the Jetronic digital fuel injection system with digital electronic spark ignition all under control of a microprocessor, and connected to an array of sensors to monitor and feedback engine parameters. A later development was “sequential multi-port fuel injection” where the computer would control the timing of each fuel injector separately to coincide with the intake valve timing.

Only relatively recently, “Gasoline Direct Injection” GDI has come into common use. In this system, fuel is directly injected into the cylinder (image on right) rather than upstream of the valves (image on the left).

This is not a particularly new idea, and was being tried as early as the 1900’s. The first production car to use direct injection was the Mercedes 300SL from 1954, however the system was not practical at the time without the techology used today.

Manifold injection requires only 2-3 bars of fuel pressure (1 bar = 14 pounds per square inch). GDI, especially with turbocharged engines, requires much higher pressure, on the order of 200 bars, as fuel is being injected into highly compressed air and there is less time to do it. For GDI, the fuel could be injected onto the cylinder walls or directly into the center of the cylinder which is how the S63top works. The latter is more advanced and allows for a finer control over the air-fuel mixture.

Here we see the centralized fuel injector with the spark plug just to the left of it.

The N63 and S63 engines without variable valve lift use newer single nozzle piezo-electric injectors. These injectors, rather than using a solenoid, use a stack of crystals that grow when an electric charge is supplied. This allows for extremely quick opening and closing. BMW calls that system HPI (High-Precision Injection).

The S63top introduced variable valve lift, and the piezo style fuel injectors were too big to fit alongside the mechanism for controlling valve lift, so smaller injectors were required. Thus a new system with variable valve lift (which BMW calls Valvetronic) and turbocharging was required. This is called TVDI, for Turbocharged Valvetronic Direct Injection.

TVDI goes back to using solenoid electro-magnetic technology, but an advanced version (called HDE) that leverages the new Bosch HDEV5.2 injectors shown below.

HDE injectors operate at very high pressure, have multiple holes in the nozzle, and are optimized for fast and short opening times.The advantages of GDI in general, and HDE in particular, is finer atomization of the fuel due to the higher pressures, less accumulated fuel on the intake valves and manifold, and a cylinder cooling effect which allows for a very high 10:1 compression ratio for a car turbocharged with over 1.5 bar of boost.

Different fuel pressure is required according to the operating conditions. Maximum pressure (p) of 200 bar (2900 psi) is only required at high engine load (m) and low engine speed (n).

The high pressures in the fuel rail are achieved using two single-piston pumps (2 & 7) driven from the exhaust camshafts by three-lobed cams. In the S63 there is a pump for each rail to ensure that sufficient pressure is generated under all operating conditions. Fuel is fed to these pumps via (8 & 1) from an enlarged (80L) steel gas tank by a 5 bar electric fuel pump adapted for the increased demands of the engine.

The illustration below shows the high-pressure fuel pump. The piston (4) compresses the fuel, directing it through a high-pressure non-return valve (2) to the fuel rail via the high-pressure connector (B). In case of over-pressure (245 bar), the pressure relief valve (3) will open. Because the piston is constantly operating, the pressure is electronically controlled by the volume of fuel allowed to enter the high-pressure system by the volume control valve (5) controlled by the DME via electrical connection (6).

GDI can alter between spraying the fuel into the cylinder on the intake stroke to maximize fuel delivery with a homogeneous mixture, or spray fuel in later towards the end of the compression stroke, directing it at an indentation in the piston head near the spark. By doing this, the air-fuel mixture can be “leaned out”, meaning that there is far less fuel than Oxygen, but still enjoy complete combustion for the sake of emissions control. This is done when the power demands on the engine are low (such as when slowing down or coasting). A GDI engine can get extremely lean, towards about 60:1 ratios, which cannot be effectively done in a non GDI car (the super lean mixture would not combust).

There are both multiple nozzles, and the fuel pressure is high enough and the nozzles are fast enough that they can be opened and closed multiple times during intake and compression strokes to create an optimal air-fuel mixture. Moreover, the fuel is at a sufficiently high pressure that it can be forced through the nozzle into the cylinder at a 10.0:1 compressions ratio (this, combined with the turbos, means the air in the cylinders gets compressed to about 20 bar or so, which means the fuel pressure must be considerably higher than this to penetrate during the compression stroke).

One potential issue with fuel combustion is “knock”. Knock is when the fuel prematurely explodes due to the pressure alone (this is actually how a diesel engine work all the time), or has multiple explosions due to an uneven mixture that does not combust all at once. This can damage an engine quickly.

The engine has “anti-knock” sensors that are essentially microphones attached to the outside of the cylinder walls that listen for explosions happening at the wrong time. These signals are sent to the DME computer that can adjust valve and fuel timing to make it stop. However, if it has to do that, combustion is no longer optimally efficient, and exhaust gasses go up, fuel economy gets worse, power is lost, and the engine can heat up more, possibly damaging the valves and the turbo bearings which are both sensitive to excessive heat.

Knock has a lot to do with the octane rating of the gas that you put in the car. The recommended gasoline octane rating for the F10 M5 is AKI (Anti-Knock Index) 93 with AKI 91 as a minimum to not impair performance. Actually “octane rating” is a misnomer. While higher octane can prevent knock, it is not the only way of doing so. So AKI is the correct term.

However, as the engine is knock controlled, the lower ratings will not damage the engine. Higher AKI ratings mean the fuel can be compressed more before detonation, which means that more energy can be extracted during detonation. An overly low AKI will require the DME to retard timing to prevent knock which is inefficient and long-term damaging. Higher AKIs are of no use, since the car cannot increase compression ratios past its design point.

The use of GDI in the new BMW turbocharged variable valve lift engines is a real step forwards in the ongoing quest to increase power while reducing emissions.