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Real World Fuel Savers

Three DIY electronic approaches that will improve your car's fuel economy

by Julian Edgar

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At a glance...

  • Automatic air conditioner controller
  • Real time air/fuel ratio displays
  • On-dash engine inefficiency warning
  • All DIY, low cost electronic modules
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This article was first published in 2009.

Look around the web using a search term like ‘fuel savers’ and you’ll find a heap of answers.

Trouble is, many of the techniques that you’ll find are doubtful at best – and at worst, could kill your car. Leaning out mixtures by intercepting the wide-band oxygen sensor, running on-board hydrogen generators, using special spark plug amplifiers, even magnets that you install on your fuel lines.

But here are three DIY electronic modules that in 99 per cent of cases will save you fuel. Best of all, they’re backed by genuine, unambiguous science – not metaphysics. Your engine's reliability will not suffer, and you won’t be running illegal tail-pipe emissions.

Let’s take a look.

Air Conditioner Controller

The air conditioner controller switches off the air con compressor at high throttle angles. As a result, over a normal driving cycle, the air con compressor operates far more often on trailing throttle, so saving you fuel. The air conditioner controller is especially effective on small engine cars.

So how does the controller work?

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The heart of the system is the eLabtronics Voltage Switch. This module is designed and built by Australian electronics company, eLabtronics. The module is internally highly sophisticated but has only four external wiring connections. The Voltage Switch is available fully built and tested from the AutoSpeed Shop - eLabtronics Voltage Switch - Prebuilt.

In addition to power and ground connections, there are ‘in’ and ‘out’ connections. In this application, the ‘in’ terminal is connected to the throttle position sensor that is standard fitment on all cars. These sensors typically output a voltage that rises from about 0.5V to about 4.5V as the throttle is opened. The Voltage Switch is able to monitor this voltage, at the same time not affecting the standard operation of the car’s engine management or electronic throttle systems.

By setting an adjustable pot on the Voltage Switch, the Switch can be made to trip at any throttle position you choose. In this application, you’d probably have it tripping at about one-third throttle.

Now, how does the Voltage Switch turn off the air conditioner compressor?

All air conditioner compressors use an electro-magnetic clutch. This means that when power is fed to the clutch, the compressor is able to be driven by the engine. When power is cut off, the compressor drive wheel just spins freely. This compressor clutch power feed is the single wire that you can see going to the front of the compressor. To disable the compressor, all that we need to do is to put a relay (an electromagnetic switch) in this circuit, so that when the Voltage Switch trips, the power feed to the compressor clutch is turned off.

To see the full detail on how to install the air conditioner controller, go to Auto Air Conditioner Controller.

We fitted the controller to a hybrid Honda Insight. The results exceeded our expectations. On the small engine Honda, the ability to hold higher gears up hills and accelerate more easily at small throttle angles – bonus outcomes - were excellent.

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Another unexpected benefit – and probably the thing we noticed the most – was the absence of the air con cutting-in when climbing a long hill. Previously, you’d have the car set to work with a fairly large throttle angle at low-ish revs in a high gear (the best approach for fuel economy) and then part way up the hill, the air con would cut-in and all the balance would be lost.

With the Auto Air Conditioner Controller in place, time after time it was like you had a really attentive passenger with their finger hovering over the air con button, ready to turn the air con off at the first sign you could benefit from their doing so. From a cabin comfort perspective, in nearly all conditions the short term ‘off’ periods of the air con were barely noticeable.

In over a year of testing, the fuel economy penalty for running the air con dropped from 20 pent cent to just 10 per cent – a major real world gain in fuel economy.

Oxygen Sensor Monitor

All cars are fitted with exhaust gas oxygen sensors designed to monitor the instantaneous air/fuel ratio. As its name suggests, the air/fuel ratio indicates the proportions of air and fuel being mixed to burn in the engine’s combustion chamber. The air/fuel ratio is often referred to as the ‘mixture’ – a rich mixture adds lots of fuel to the air, while a lean mixture adds little fuel to the ingoing air.

  • Narrow Band Oxygen Sensors

Most cars of the last 20+ years are fitted with what are called narrow band oxygen sensors. Typically, they take the form of a zirconia device that, in the presence of exhaust gases, generates its own output voltage. If you like, it acts like a little battery. The sensor outputs a voltage of approximately 100 - 900mV (ie 0.1 – 0.9V), depending on the mixture strength.

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In most cars, the oxygen sensor is used by the ECU to help maintain an air/fuel ratio of about 14.7:1 during idle, light load and cruise conditions. This behaviour is called ‘closed loop’. This diagram shows the closed loop functioning.

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Because it is a feedback loop, the actual mixtures constantly fluctuate a little above and below stoichiometric. The voltage output of the sensor therefore also fluctuates.

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The easiest way of monitoring the output of a narrow band sensor is with a LED Mixture Meter. These meters simply display the raw output voltage of the sensor, with the illuminated LED indicating the voltage.

As described above, in closed loop running the output of the oxygen sensor will rapidly rise and fall as the ECU tries to keep the air/fuel ratios around 14.7:1. Thus a flickering Mixture Meter display indicates closed loop operation. On the over-run, the ECU will switch off the injectors (ie the Mixture Meter LEDs will all go out, indicating a very lean mixture), while at full throttle the mixtures will be enriched, something that the Mixture Meter display will show by lighting a LED at the ‘high voltage’ end of its display.

So a simple narrow band LED Mixture Meter will indicate the operation of closed loop, injector shut-off, high load enrichment, health of the oxygen sensor, and the presence of some mechanical and electrical engine management problems – eg the fuel pump getting tired.

It will also allow you to drive more economically – more on this in a moment.

Go to Monitoring Factory Oxygen Sensors, Part 1 for the full detail on building and fitting a narrow band oxy sensor monitor.

  • Wideband Oxygen Sensors

As their name suggests, wideband sensors operate over a much broader range of air/fuel ratios than narrow band sensors. This is especially important in cars that run ultra-lean cruise capability, where the air/fuel ratio must still be monitored by the ECU when it is beyond the measuring capability of narrow band sensors. The electronic action of a wideband sensor also differs from a narrow band sensor.

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Wideband sensors can be monitored using another electronic kit – an Automotive Voltage Monitor. The Automotive Voltage Monitor can be found here Voltage Monitor Kit.

What makes the bargraph kit particularly suitable for this application is that the voltage at which the top and bottom LEDs light up can be independently set, the display then automatically scaling the in-between LEDs into ten equal steps.

The Automotive Voltage Monitor kit uses rectangular LEDs mounted along one side of the board. The kit can therefore be mounted in a small box, with the LEDs protruding through a slot cut in the side of the box. Like the narrow band Mixture Meter, the completed Voltage Monitor assembly is small enough to mount on top of the steering column, or in some other convenient place on the dash. Alternatively, you may choose to remote-mount the LEDs, connecting them via ribbon cable to the module.

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Another approach is to do what we did, and that is to remote-mount only a couple of the LEDs (arrowed). In the Honda Insight in which the system was installed, it was decided that only two conditions needed to be seen. The first was when the car was in lean cruise – that is, running with an air/fuel ratio of about 25:1 (no mistake – it really does go that lean!). The second condition was when the engine management dialled-up a very rich air/fuel ratio – about 12.5:1. (The third LED shows the action of a completely different item.)

Go to Monitoring Factory Oxygen Sensors, Part 2 for more detail on the technology behind wideband sensors and how to connect a wideband display to your car.

  • Using Oxygen Sensor Displays

So how do you use the oxygen sensor displays to improve fuel economy?

The greatest gain in fuel economy will be made in most cars if the fuel enrichment that occurs at high loads is rarely seen. For example, when climbing a hill, you might find that lifting your foot just a fraction causes the ECU to switch back to closed loop from enriched mode. Lifting your foot may make almost no difference to your speed up the hill, but will reduce fuel consumption.

Moving to higher gears earlier may result in the car staying in closed loop; in other cars it will be better to rev slightly higher at a reduced throttle angle before changing up.

In some cars the engine over-run injector shut-off does not function unless revs are above a certain level. When travelling down a long hill with no throttle being applied, in these cars it can be better to drop down a gear, so lifting no-load revs and triggering the injector shut-off.

In other cars, the clear indication of when the car is in lean cruise allows the driver to better keep the vehicle in that mode – to put this another way, it’s much easier to see in what conditions the car enacts lean cruise and in what conditions it leaves this mode.

Being able to see in real time what mixtures the engine is using is an invaluable window into engine operation, allowing much more economical driving to occur.


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FuelSmart is an electronic module that monitors two engine management sensors - the throttle position sensor and the MAP sensor. By doing this it can sense when the engine is working in a way that is likely to result in poorer fuel economy. Specifically, when the throttle is open but the MAP sensor indicates that engine vacuum is above a preset level, a dash LED is illuminated.

In technical terms, FuelSmart indicates when the engine’s Brake Specific Fuel Consumption (BSFC) is likely to be high because of large pumping losses resulting from the engine trying to draw air past the partly closed throttle butterfly.

Go to FuelSmart Part 1 and FuelSmart 2 for more on installing FuelSmart. FuelSmart is available directly from Australian electronics company eLabtronics

So how do you use FuelSmart? If the FuelSmart dashboard LED is on, you should immediately assess your driving style.

For example, when going up through the gears, if the LED stays on, you should use more throttle between each gear change. If you are slowing for a set of traffic lights and the LED stays on, you should lift right off – not trail along with the throttle just open.

In stop-start urban traffic and hilly areas (amongst the most difficult of conditions to get good fuel economy), you can use the LED indicator to guide you in variations of ‘pulse and glide’. (Pulse and glide is where you accelerate in the zone of max engine efficiency and then lift right off – it works well in hilly areas and many heavy urban traffic situations, where the throttle is usually on/off a lot.)

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In constant load cruise conditions, the FuelSmart LED will stay on – but that’s because in these conditions the engine is actually not working at its fuel-efficient optimum. If you are already in top gear, there’s nothing much more you can do. But if you’re not in top gear, the LED is a reminder to change up and then use more throttle to maintain speed.

The improvement in fuel economy depends on a number of factors – the terrain, average speed, constancy of that speed, the ability of the driver and how the FuelSmart module is calibrated. It also depends on the car. However, in back-to-back testing over the same route, we saw a repeatable 15 per cent fuel economy improvement – and that comparison was with a well-driven car!

By watching the LED and driving accordingly, a driver unused to driving for fuel economy could gain far more improvement than 15 per cent.


None of these electronic modules is as sexy as magnet that you put on your fuel lines to give an immediate 20 per cent improvement in fuel economy. Or complex hydrogen production systems that turn your car into a dual fuel economy monster. But the difference is that these modules are relatively cheap, based on scientifically rigorous concepts – and actually work!

The Air Conditioner Controller requires no driver effort and can result in a very good summer fuel economy improvement.

The effectiveness of the Oxygen Sensor Monitor depends on the car and the driver. In some cars, eg those with lean cruise capability, the driver can very successfully use the LED indication to stay out of enriched fuel modes and remain longer in lean modes. Watching the display as you drive the car soon gives you a good feel for the way the car’s engine management system provides different mixtures; the next step - driving with regard to those mixtures - is a pretty easy one.

The FuelSmart module has the best potential for improved fuel economy – but that’s because it can require the greatest change in driving behaviour. Drive strictly according to the FuelSmart indicator and you’ll be working pretty bloody hard! But that’s not an absolute requirement: the LED indicator is just a warning that you should assess how you’re driving.

Each of these approaches improves fuel economy – having all of them installed is best!

The Fun of Driving for Fuel Economy

Over the last few years a number of websites devoted to people chasing best fuel economy have appeared. Modifications, driving techniques and best economy cars are discussed in detail.

At first glance these enthusiasts seem to be diametrically opposed to traditional car nuts: after all, what could be more boring than gaining the best fuel economy?

But in these times of increasing fuel costs and road rules that are getting ever more tightly policed, challenging yourself to achieve best fuel economy is one of the few remaining ways you can have fun on the road without looking at potential jail time.

That’s especially the case if your car has a fuel consumption display. Reset it before your daily commute and see how low you get can the fuel consumed. Set yourself the challenge and you’ll be amazed at how interesting what would otherwise be a boring daily task can become.

Doing well requires high order car control and handling skills, the ability to read traffic and other variable conditions, and intense mental concentration. And, rather like traditional performance driving, the skills for the open road are different from those that work best in urban areas.

I will never lose a love of driving hard and fast, but when - in nearly all public road driving - that is now impossible, striving to gain best fuel consumption is a very rewarding substitute.

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