CONTINUOUS INJECTION SYSTEM
Once again, Porsche collaborated with its technology partner and Stuttgart neighbour, Bosch. The pair had been developing an electronic alternative to mechanical injection since 1967 and Bosch’s K-jetronic was the second version tried on a Porsche. An early D-jetronic had been devised for the Vwengined 914, a model which would sell largely in the US. K-jetronic, which stood for Kontinuierlich, was so named because unlike earlier Bosch mechanisms, flow through the injectors was continuous. Whereas the preceding MFI operating at 250psi fired its finely atomised squirts according to the engine’s ignition timing, K-jetronic worked at 80psi, adequate in terms of atomisation, but by spraying the intake ports continuously, some fuel would always remain until the next opening of the intake valves. This promoted a special tumbling effect, a better mixing of air and fuel. While this was to the detriment of out-and-out performance – CIS engines did not have the same high-revving enthusiasm of the Mfi-equipped engine – it wasted less fuel, burnt what it needed more thoroughly and consequently emissions were much cleaner. So much better in fact that Porsche was able to fit the catalytic converters which would become mandatory in the US from 1975.
Instead of the MFI’S energy-absorbing mechanical pump, K-jetronic used an electric item. Bosch engineers had tried various devices to meter engine air intake and finally settled on a disc carried in a conical orifice through which air was drawn from the narrow end to the wider. A counterbalancing arm transmitted the intake disc’s movements to a fuel distributor unit. This received atomised petrol
(at 80psi) which it then delivered to a control cylinder with a narrow vertical slit for each of the six cylinders. Inside this control cylinder a plunger was moved up and down by the arm from the metering disc, with the plunger’s precisely machined edge deciding how much fuel would be released through the slit and on through a control valve to the injector nozzle. The operations were overseen by solenoids and a transistor.
Complicated to describe, but in operation relatively straightforward, K-jetronic was easier to set up and maintain than MFI which had large numbers of moving (and wearing) parts. There were initial learning curve problems: Porsche found that using the port intake nozzle position of the MFI often caused hot starting reluctance, so the nozzles were later moved slightly to a cooler spot while fuel pressure for starting was maintained by an accumulator. Often criticised for taking the performance edge off the 911, K-jetronic was remarkably effective: by comparison, British sportscar makes lost a great deal more power, the federalised TR6 losing 30 of its 150bhp when forced to abandon its Lucas MFI, and the American muscle cars completely hobbled by emissions regulations.
As Porsche gradually honed the 911’s injection, by 1981 power was back to MFI levels and torque substantially better. The 3.2’s Le-jetronic saw electronic injection make further advances in 1983, introducing fuel cut-off on overrun, and a new ECU programmed with several hundred road speed/ engine temperature variations kept the flat six working just below the knock limit (pre-detonation), allowing a higher 10.3:1 compression ratio for more efficient combustion; the 3.2 also offered a 10 per cent improvement in mpg over the 3.0 SC.
The advent of the 964 heralded a further advance in Bosch injection management, the new system clearly related to those used by Porsche today. Now called Motronic, its central processor or DME (digital motor electronics) reflected the march of software and was developed in conjunction with the 964’s redesigned air intake as Porsche started to focus on cylinder filling and the engine’s medium and high rev volumetric efficiency; sequential rather than ‘batch’ fuelling was overseen by the Motronic and the new DME added a downloadable fault-reporting function. The 993’s engine was essentially a refinement of the 964’s, the DME developed in line with the 993’s more sophisticated air-intake metering.
On water-cooled Porsches, the complexities of engine management systems have become such, monitoring hundreds of parameters every second, that it is no longer feasible to describe injection in isolation. Bosch again helped develop the most significant development to fuelling change, namely the switch to direct injection for the 997.2 generation. This is the first injection system to surpass the original MFI’S level of atomisation and has the advantage of squirting straight into the combustion chambers rather than an adjacent plenum. Expensive and requiring far more powerful fuel pumps and a bigger electrical loom, direct injection allows higher compression ratios (now 12.5:1, 12.9:1 for the GT3) which improves economy (and reduces CO2). Like the old MFI, very high-pressure DFI pumps need fuel for cooling and lubrication too, and US reports suggest that since 2011 Porsche has modified the pump design several times (like the IMS bearing in the mid-2000s) after instances of failure, though these sources add that other manufacturers’ DFIS have known greater failure rates.
On the 992, which features a significantly reworked flat six, Porsche uses ‘Piezo’ injectors previously found on diesel injection which further refine injection precision, while adjustment of part-throttle intake valve lift promotes better integration of the mixture. Driven by the need to get more mpg and power from less fuel (and by politicians’ obsession with carbon dioxide) Porsche’s ingenuity appears to have no bounds.