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On-demand fuelling

The best businesses, so we are told, are those that provide customers with precisely what they want at precisely the time they need it. ‘Just in time’ was the mantra of the 1990s automotive industry, and with it came the complexities of scheduling and the task of ensuring things turned up at the production line at the correct time and in the correct order.

But the supply side of modern manufacturing is not the only place where the supply needs to be matched to demand. Another place, one perhaps of more interest to RET-Monitor readers, is in modern automotive fuel systems, where just sufficient fuel as is needed has to be pumped out of the tank to the exact pressure required at just the right time – no more and certainly no less.

In the more traditional manifold or port-injected system, the fuel is pumped into a simple rail, at the end of which lies a fuel pressure regulator. Once the fuel is up to the pressure required, any excess flows through the regulator valve and returns to the fuel tank, taking with it any heat absorbed during its passage across the engine. Accurately metering the fuel these systems requires a constant fuel pressure difference across the injector, and to maintain this the fuel pressure regulator is linked to the intake manifold by a simple tapping that alters the fuel pressure setting by the amount equivalent to the depression in the manifold. A highly functional system, you may say, but in supplying more fuel at pressure than is strictly necessary and then spilling it back to the fuel tank it is surely grossly inefficient.

A much more efficient way that complies with the ‘just in time’ principle is the demand-controlled system. Dispensing with the pressure regulator as such and replacing it with a closed-loop control system monitoring fuel rail pressure ensures that just enough fuel is delivered to the engine at the right time – and, perhaps more important, at the pressure desired. In compressing only that fuel necessary to feed the engine, engine efficiency is improved, and rather than controlling the fuel pressure difference across the injector mechanically, it can be controlled much more simply and electronically via the engine’s ECU. 

Of course, we still need things like a pressure relief valve should things fail, but this can be packaged inside the fuel tank along with the pump. Arguably, however, the biggest advantages of these types of systems are their adaptability and ability to increase fuel pressures if required. When an engine is hot, for instance, and the heat soak into the fuel rail at, say, a pit stop generates a fuel vapour lock (increasing the fuel rail pressure) then these systems can be made to condense the fuel vapour bubbles, enabling the engine to inject fuel and fire again. Also, in turbocharging applications, the metering range of the fuel injectors can be widened by changing the pressure difference across the injector – increasing it for full-load, wide-open throttle running and reducing it when much lower power is required or when the engine is at idle, effectively improving the precision of the fuelling.

Fuel at the right pressure delivered just at the right time? What more can you ask for?

Fig. 1 - Fuel pressure regulator for a port-injected engine

Written by John Coxon

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