Performance Electronics, Part 2

Last week in Part 1 we introduced systems that switch at a set-point. Examples include intercooler water sprays that turn-on at a preset boost pressure, shift-lights that glow when revs get near the redline, and warning lights that turn on when engine oil temperature is too high. In addition to set-points, we also covered hysteresis (the difference between the switch-on and switch-off points) and looked at situations where using the right level of hysteresis was very important in determining how well the system actually worked.

But what about a system where you don’t want the device on or off – you want it to progressively vary between these two extremes? That’s a very common requirement in cars. Think about the flow of fuel through the injectors, the flow of air through a boost pressure control solenoid, or the flow of fluid through an auto trans line pressure control valve. In all these cases, the flow has to be variable across a wide range – rather than being simply on or off.

So how do these variable flow systems work – and how can they be modified?

Fuel Injectors

Let’s start with the system most people are likely to be familiar with – the electronic fuel injection. In the fuel injection system, the fuel is supplied at high pressure to injectors. Injectors are just solenoid valves with a built-in fine nozzle. When power is applied, the injector pintle rises, letting fuel flow through the nozzle in a spray. When power is removed, the nozzle shuts, stopping the flow of fuel.

So yes, just like last week’s systems, the injector is either on or off. However, if we pulse the injector fast, we can achieve something else that’s very important - a variation in flow.

When the engine is spinning at 2000 rpm, there are about 16 intake strokes every second. Since we add fuel every intake stroke, at 2000 rpm we need to fire the injector (and so squirt in a bit of fuel) 16 times a second. Rather than write "times a second", we say the injector is being pulsed at 16 Hertz. This is the injector’s firing frequency.

Frequency refers to how many times something occurs per second. It is expressed in Hertz (Hz).

If we have to open the injector 16 times a second, we obviously have a maximum of 1/16^th of a second to get the injector open, squirt out some fuel, and then close it again, ready for the next event. It sounds a short time, but for an injector, 1/16th of a second is long enough to take a holiday in the sun. So at these revs, it’s likely it will be open for only a small proportion of the available time – say 10 per cent of the 1/16th of a second. This percentage is called its duty cycle.

Duty cycle refers to the proportion of available time the solenoid valve is open for. It is expressed in per cent.

If the duty cycle is at 50 per cent, the injector is open for half the time. If the injector is at 75 per cent duty cycle, it is open for three-quarters of the available time. At 100 per cent duty cycle it is open continuously, while at 0 per cent duty cycle it is continuously shut.

You need to remember that the lift of the pintle is the same irrespective of the duty cycle; it’s the duration of its opening that’s important. It’s also vital to remember that at other than 0 and 100 per cent duty cycles, the injector is opening and closing at its pulsing frequency, which varies with engine speed.

Slow Motion It’s hard for humans to think of what’s happening all in a blink of an eye. So let’s s-l-o-w it right down. Instead of an injector, think of a bouncer (er, crowd control officer) out the front of a club. He’s opening and closing the door to let the queued-up line of people through. But this club is a weird one, because he swings opens the door only once every minute. To make sure he remembers to, there’s a buzzer that sounds at one minute intervals. The rules might say he opens the door every minute, but he decides how long it stays open for. When he’s feeling a bit of a bastard, he opens it for only 5 seconds out of each 60. In other words, he opens it when the buzzer sounds and closes it just 5 seconds later. He’s therefore has it open for 8.3 per cent of the time. On other nights, when he’s feeling a bit generous, he opens it for 30 seconds out of the available 60 seconds (a 50 per cent duty cycle). Once, long-time patrons remember him opening the door for 45 seconds out of each 60 (a 75 per cent duty cycle) but that hasn’t been seen for years. So the door is being operated with a fixed frequency (once per minute or 0.017 Hz) but with a duty cycle that varies between 0 (door always shut) and 75 per cent (door open for 45 seconds out of the available 60). You can see now that it’s not very hard to picture a fixed frequency, variable duty cycle. And a variable frequency is pretty easy as well – just change how long it is between each sounding of the door-opening buzzer. (A point to think about is that a 15 second door opening time at one-a-minute frequency gives a duty cycle of 25 per cent, but that same 15 second door opening at a two-a-minute opening frequency has increased the duty cycle to 50 per cent. That’s why the measured duty cycle of injectors goes up so fast with rpm – there’s less time to get in the fuel.)

Other Solenoids

In the case of an injector, the pulsing frequency must vary with engine rpm, otherwise it won’t be able to add fuel each intake stroke. However, there are lots of other flow control valves in a car where the pulsing frequency can stay fixed, with just the duty cycle varying.

For example, a boost control solenoid might have a frequency of 10Hz – it opens and shuts ten times a second. Like an injector, its duty cycle will be varied within its 1/10th of a second windows of opportunity, being able to be at 0 per cent duty cycle (closed), 50 per cent duty cycle (open for half the available time), or 100 per cent duty cycle (continuously open). In this case, the frequency can be picked to suit the characteristics of the solenoid valve, with a potential huge advantage.

Most variable duty cycle solenoid valves other than injectors don’t completely open and close when operating. Instead, the pintle tends to hover in mid positions – it’s being pulsed so quickly that it hasn’t got a chance to get to either end. Instead, the strength of the magnetic field caused by the coil varies progressively, and so the valve opening also varies progressively. There’s no clicking on and off of the valve, because the pintle is hovering at mid-positions.

These valves must use a fixed frequency, one that gives this ‘hovering’ result. If the pulsing frequency is too high, the valve might not even move – it doesn’t know where the bloody hell it’s meant to be. If the frequency is too low, it’s ponderously opening and closing, shortening the life of the valve as its pounds its seat and sometimes also not progressively controlling the flow through it as finely as it should.

Mix and Match

So a valve being pulsed to control the flow of a fluid can be operated in two ways. The first method is a variable frequency, variable duty cycle – as is done with the fuel injectors. The second approach is a fixed frequency, variable duty cycle – as is done with most of the other flow control solenoids in a car.

In Brief: The Modification Implications All this used to fall into the yeah-it’s technically-trick-but-so-what? area of knowledge. But the release of two kits designed by Silicon Chip magazine has changed all of that. The first is the Digital Pulse Adjuster and the second is the Independent Electronic Boost Control – and the latter can be used to control a lot more than boost! The Digital Pulse Adjuster (DPA) is purpose-designed to allow the modification of the duty cycles with which a solenoid valve is triggered. It intercepts the signal coming from the Electronic Control Unit (ECU) and allows the user to shorten or lengthen the duty cycle that’s going to the valve. But what about the pulsing frequency? What does it do about that? The short answer is nothing – it maintains whatever pulsing frequency was originally being used to drive the valve, even if (like with injectors) that varies. In other words, the valve doesn’t see anything different in the way it’s being operated – it’s just that its duty cycle is now fully controllable by the modifier at 128 points. This is achieved by the user mapping how much they want each of the original duty cycles altered by, either up or down. On the other hand, the Independent Electronic Boost Control (IEBC) maintains a fixed output frequency and the user sets the percentage output duty cycleshe or she wants.   Digital Pulse Adjuster Independent Electronic Boost Control Input signal Original solenoid control signal Original solenoid control signal Output signal Original solenoid control signal plus the up/down duty cycle adjustments User-set duty cycles Pulsing frequency Maintained as original Fixed, normally at 10Hz User Control 128 sites Two selectable maps, each of 64 sites With these two modules, you can do simply awesome things with any car system running variable duty cycle solenoid valves. That includes modifications to turbo boost, power steering weight, idle speed, and auto trans line pressures. Added systems can include extra injectors and water injection systems. See Digital Pulse Adjuster, Part 1 and Independent Electronic Boost Control story for more on these two modification tools. The book which overs both these projects and a host of others is High Performance Electronics for Cars .

Conclusion

The modification opportunities are fantastic, but if you don’t know what duty cycle and frequency mean, you’re sure as hell going to be very lost... Maybe a good idea to read this article again?

Next week: more on digital pulsing

Independent Electronic Boost Control
Digital Pulse Adjuster
High Performance Electronics for Cars

Pulse Width Modulation In this story we’ve talked about varying the duty cycle to alter how much a valve flows. Another term for systems that use variable duty cycles is Pulse Width Modulation (PWM).