Advances in electronics and computer control enhanced the core advantages of fuel injection in classic Porsches, elevating their performance and efficiency to new heights...
Porsche’s use of electronic fuel injection.
In the previous instalment of our series focusing on the fuel preparation technology of air-cooled Porsches, mechanical fuel injection evolved into the advanced Continuous Injection System (CIS) from Bosch, which remained in use up to the end of 964 production. As the name suggests (it was also called K-jetronic), CIS continuously injected fuel into the inlet ports, which came with the disadvantage there would often be unused fuel remaining in the runners, which is bad for emissions and fuel economy. Nonetheless, CIS did pave the way for full electronic control of fuel injection.
In fact, electronic fuel injection (EFI) pre-dated the CIS in Porsche’s cars. The 914, launched in 1969, was the first Porsche to use EFI. Its Volkswagen Type 4-derived flat-four was fitted with Bosch D-jetronic system (D for druck in German, meaning pressure in English). Indeed, Porsche considered using the same system on the
911, but it was decided CIS was better-suited to the high-revving flat-six. The 1.7-litre boxer in the 914 was a different beast, producing peak power at just 4,900rpm.
Bosch licensed an EFI design named Electrojector from the American Bendrix Corporation, and this is what the D-jetronic system was based on. The control unit for the injection system was rudimentary by today’s microprocessor standards, relying on ‘analogue’ circuitry and expensive and unreliable sensors for its input signals. Its logic was based on the mathematical speed-density air mass flow model, which calculates the air mass flow rate into an engine using various inputs, including the inlet manifold pressure. The intake air temperature is measured because air density increases as its temperature decreases and the engine speed is critical to the calculation, too. In the 914’s engines, the latter was picked up by a pair of trigger points at the base of the ignition distributor shaft, though these are prone to wear, reducing the effectiveness of the system.
The 1.7-litre and two-litre flat-four engines in the 914 use a single throttle body feeding an intake plenum positioned top-centre of the engine. Two inlet runners on each side then feed the air to the cylinders and it’s into
these runners the injectors are mounted. A short fuel rail on each bank supplies continuously pressurised fuel to the injectors, which each feature an electromagnetic solenoid. Unlike CIS, these don’t open when the pressure reaches a certain level — their opening and closing is strictly controlled.
When the engine is running, the positive terminals of the injectors are supplied with voltage. Then, to open the injectors, a transistor in the system’s control unit switches the negative terminal to ground, completing the circuit. The injected quantity of fuel is varied by the amount of time the injector is open for and the required quantity is determined using the calculated air mass flow, the desired air-fuel ratio and a reading from the analogue throttle sensor. Other inputs affecting the control system include a barometric pressure input (to compensate for altitude changes) and a cylinder head temperature sensor. The reading from the latter allows the control unit to enrich the air-fuel ratio for cold starts and while warming up.
The D-jetronic system didn’t feature the sophistication to allow for sequential injection, where each of the injectors are opened in sequence, aligned with the firing order of the engine. Instead, it opened alternating pairs of injectors each revolution of the engine. This could lead to fuel pooling on the walls of the intake ports, but to a lesser extent than with the earlier Continuous Injection System. Sequential injection was still some time away, but Bosch’s next-generation EFI system had more sophisticated electronics and came to the 914 in 1974, when the 1.8-litre flat-four replaced the 1.7-litre unit. The new engine was fitted with the L-jetronic system. The L stands for luft in German, which translates as air in English. This signified a change in strategy to obtain the flow rate of the air entering the engine.
Where the D-jetronic system relied on a calculation using various inputs, L-jetronic directly measured the flow. It did so with a spring-loaded vane sitting in the inlet tube upstream of the throttle. The higher the airflow, the more this vane was pushed down, taking with it another arm in the meter. The position of this arm governed the electrical resistance and hence the voltage to the control unit, allowing more reliable determining of the airflow rate (calculated using other inputs). It could do without a throttle position sensor, too, while the inlet air temperature sensor was integrated into the airflow meter. Meanwhile, cold-start warm-up was sped up by extra airflow allowed through a ‘thermotime’ switch, opened when cold using a bimetal strip. This was a purely mechanical device, unconnected to the control unit. However, the extra air it allowed in prompted the control unit to increase the amount of fuel injected, causing faster idle for a fixed amount of time.
The L-jetronic concept was further refined through the years, notably improved and miniaturised as computing power advanced at speed. The next major advancement in electronic fuel injection, however, was to bundle its control together with that of the ignition system and closed-loop feedback, using oxygen sensors. Bosch launched its first digital engine control unit in 1979, naming it Motronic. The first Porsche to use this system was, in fact, the 944 of 1983, but from an air-cooled perspective, Motronic arrived on the 1984 Carrera
3.2 and has been referred to as DME (Digital Motor
MIXING FUEL AND AIR IN THE RIGHT PROPORTIONS AT THE RIGHT TIME IS KEY TO THE OPERATION OF ANY ENGINE
Electronics) ever since. This is probably a good time to explain open-loop and closed-loop control. Put simply, during open-loop control, the control unit for the engine follows a set of instructions for the fuel injection based on a relatively simple number of inputs, such as throttle position and engine speed. Closed-loop control is more sophisticated.
The control ‘loop’ is closed by a lambda sensor (or sensors) in the exhaust system. This is also referred to as an oxygen sensor (or simply an O2 sensor). It measures the proportion of oxygen in the exhaust stream, from which the control unit can determine the air-fuel ratio. As detailed in our first article on fuel mixing in air-cooled Porsches (back in issue 77 of Classic Porsche, which you can order at bit.ly/issuescp), where we went through the design of carburettors, mixing fuel and air in the right proportions at the right time is key to the operation of any engine. While petrol is a volatile liquid, it needs to be mixed with air in the right proportion to ignite. The air-fuel ratio in an engine is the ratio of air to fuel in terms of mass. The ‘perfect’ ratio — defined as exactly the right amount of air to completely burn all the fuel — is called a stoichiometric mixture. For petrol, this is 14.7:1, which means that, for every gram of petrol, 14.7 grams of air is required. Lean operation is when the air-fuel ratio is higher than 14.7, and rich operation is when it’s lower.
Now, lambda equals 1.0 when the air-fuel ratio is 14.7:1, less than 1.0 for rich mixtures and greater than 1.0 for lean mixtures. For catalytic converters to be effective, lambda must be equal to 1.0 (or no more than a little less). And that’s why the lambda sensor is required. It’s all well and good having a detailed engine calibration taking variable inputs from things like air pressure, coolant temperature, ambient air temperature and throttle position and then spitting out an ideal amount of time each injector should be open for, but this is nowhere near as precise as a closed-loop system informing the control unit what the result of it all is. Naturally, the control unit also uses those other variables to make its calculations, but the lambda sensor is key to ensuring reduced emissions. Actually, later versions of CIS in the 911 featured a simple closed-loop system using an oxygen sensor. For Motronic, however, it was an integral component and a more advanced lambda sensor with heating was employed. The material of the sensor needs to get up to temperature before it predictably reacts to oxygen in the exhaust stream. The longer it takes to do this, the more time the engine runs on open-loop control. Integrating a heating element into the sensor greatly cuts down this time, contributing to much lower emissions during the engine’s warm-up phase.
STROKE OF LUCK
The Carrera 3.2 came about from the so-called “E-program” to reduce emissions and fuel consumption in a bid to meet stricter legislation in Porsche’s largest global retail markets. Beyond the increase in capacity, there were far-reaching changes to the flat-six. The latter
was achieved by using the 911 Turbo (930)’s stroke of 74.4 millimetres, in conjunction with the 95mm bore of the three-litre engine in the outgoing 911 SC. The addition of the Bosch Motronic system is credited with a ten percent reduction in fuel consumption, despite a ten percent increase in power. Some of this was achieved through a fuel cut-off feature on the overrun. For the 964, a 3.6-litre engine was developed, using dual-ignition.
This caused issues during development — the Motronic system couldn’t compute quickly enough to keep up with rising revs. The result was a significant upgrade to the Motronic control unit, which had the ability to store fault codes for the first time and took over management of an activated charcoal canister, too. This is used to prevent fuel in the tank from evaporating to atmosphere. Instead, it’s captured in the canister when the engine is disabled, the Motronic system actively extracting fuel from the canister when the engine is up and running.
The last of the air-cooled 911s, the 993, continued with the Motronic/dme control system, though with even more significant upgrades. The most important was the addition of proper mass airflow sensing. As described above, the all-important air-fuel ratio is a relationship in terms of the mass of each parts of the mixture, not the volume. Even the vane-based system introduced with L-jetronic is an indirect measurement of the mass airflow rate. It was, however, was susceptible to damage and wear. On top of that, it restricted the airflow coming into the engine. A hot-wire airflow meter was the solution, and it’s a component remaining in widespread use today. The intake air flows through a tube and, within it, is an electronically heated wire. The control unit monitors how much electrical energy is required to maintain a constant temperature. Higher airflow cools the wire, for example, necessitating higher energy. It’s relatively simple to calibrate these meters for mass flow rate, they’re robust and cause very little restriction to the incoming airflow. The next stage of advancement was direct injection into the cylinders, used today on all of Porsche’s cars, but never mass-produced in the classics. Even so, the concept goes all the way back to the days of the 356, proving the ideas and ingenuity of the engineers from Porsche’s early days are still influencing the way the brand’s cars are manufactured today.