Feeding the beast

Wednesday, July 09, 2014

Tags :  fuel-system

The fuel injector is an electromagnetically controlled device designed to meter fuel to the engine, and at the same time offer it in a form such that it can be readily combusted. Presented in this way, the task of the injector would therefore seem to be fairly straightforward, and over the years a number of designs have been offered. Pintle-type injectors, ball-and-seat designs through to simple disc derivatives – each new design has been slightly more tolerant to the build-up of carbon deposits or varnish than the one before.

But while the science of injector fouling is now well understood, the one thing that can increasingly defeat many a design is its hot fuel handling capability. And if you consider that the role of the fuel in contemporary injector architecture is one of cooling then this would seem to be a major failing.

In most commonly used injectors these days, the fuel arrives via the pump and filter and accumulates in a fuel rail under pressure. In response to the ECU-activated signal energising the solenoid, and thus opening the injector, the fuel flows axially down the injector through a simple strainer and around the solenoid coil, thus cooling it. So long as the fuel continues to flow and is injected into the cylinder, all will be well, but when the fuel stops flowing then any residual heat soak from the engine or the injector solenoid will migrate into the fuel still sitting in the injector.

As the temperature of this fuel rises then fuel vapour will form, causing a phenomenon called vapour lock, making restarting the engine very difficult. With the desire for engine packages to become much smaller, and as engine performance levels increase, this extra heat and the proximity of components such as exhaust-driven compressors will make the issue of vapour lock even more critical. In future therefore, hot fuel handling could be a major issue in low-pressure port injection systems.

Fear not though, all is not lost. One way around the problem is to increase the pressure in the fuel at restart. This should condense the fuel in the injector back into its liquid form and enable the engine to start again as planned – and indeed, most current vehicle systems work this way. Another way though, and one that is highly attractive in liquid-fuelled LPG applications, is to use so-called ‘bottom feed’ injectors.

Here, instead of feeding the fuel into the top of the injector and clamping the injector between cylinder head and the fuel rail, as is normally the case, the fuel intake into the injector is via a series of drilled holes through its side towards the bottom. Flowing through a gallery, perhaps even integrated with the cylinder head, the fuel passes into the injector and is injected out into the cylinder without going anywhere near the controlling solenoids, and any vapour (which is less likely to be formed anyway) can be easily vented and out of harm’s way. This type of system works best when the fuel is re-circulated back to the tank for cooling, and will be essential if using LPG-type fuels that boil at temperatures only slightly greater than ambient at rail pressures of about 8-10 bar.

Bottom-feed injectors were typically used on throttle body systems many years ago, but with the increasing interest in alternative fuels could they be another case of ‘back to the future’?

Fig. 1 - A bottom-feed injector

Written by John Coxon

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