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Thread: Thought for better fuel economy?

  1. #1
    Member neil's Avatar
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    May 2006

    Default Thought for better fuel economy?

    Would this be possible or feasible assuming that it's not already part of EFILive's functionality, if not would it be able to be incorporated via the software.
    I know that this article is referring to a Honda hybrid but the concept might have some parallel.


    At a glance...

    * Filtering the throttle position sensor signal to...
    * ...keep the car longer in its...
    * Incredible standard lean cruise mode...
    * ...and improve driveability
    * Very simple and cheap modification

    This is one of the most unusual modifications we’ve ever covered. It’s also amongst the cheapest and (quite incredibly!) improves the fuel economy of the most economical production car ever sold.

    (And the mod possibly has application in other cars as well.)

    This process is crystal clear on the MoTeC air/fuel ratio meter. The car stays in 25:1 lean cruise for a while (or even, as the pic shows, closer to 26:1!!), then periodically switches out of it, running 13.5:1 (ie slightly rich) air/fuel ratios for a short time before then returning to lean cruise.

    It was Lachlan Riddel of ChipTorque who made the suggestion.

    I had been talking to him about the Insight’s operating system and had suggested that I wanted the car to stay in lean cruise much more often that it did. I went through the options that had occurred to me – intercepting the (two!) oxygen sensor signals, disabling the oxy sensor feedback and controlling mixtures directly, and many other ideas. I also talked about tweaking the electric assist. Lachlan leant back in his chair and cut straight through to the fundamental.

    “I think that you should intercept the throttle position sensor signal,” he said.

    “Does the ECU use delta throttle angle or absolute value to switch out of lean cruise? If it’s just delta throttle angle [ie rate of throttle change] , all that you need to do is smooth this out.”

    What he meant was that if the car didn’t see sudden throttle movements, it might stay longer in lean cruise. I went away very thoughtful – and did some testing.

    Watching the MoTeC meter and driving the car clearly showed it was definitely the rate of throttle change (rather than absolute value of throttle position) that triggered the drop-out of lean cruise.

    Having learned of this, Lachlan would have rustled in a box and produced a ChipTorque Xede interceptor to make the changes (although I am not entirely convinced the Xede could do it anyway), but I thought of a much cheaper and simpler approach. (Sorry Lachlan!)

    Smoothing of a signal can be achieved as easily as using just a resistor and a capacitor. The resistor (as its name suggests) resists the flow of electricity, while the capacitor stores electricity.

    First, here’s the standard throttle position sensor (TPS) circuit. The TPS comprises a potentiometer (“pot”). One end connects to a regulated 5 volts, the other end to ground. The wiper (whose position varies with throttle angle) can then feed the ECU any signal from 0 to 5V, depending on its position. (In fact the range is less than this, but you get the idea.)

    But what if we change the circuit a bit? In this modified circuit, the capacitor stores electricity and so smooths the throttle fluctuations seen by the ECU. The resistor limits the speed at which the capacitor initially charges when the car is turned on and also partly isolates the ECU from the real signal.

    To tune the action of the filter, we can replace the resistor with a pot wired as a variable resistor. With this pot at one end of its travel, the TPS signal is unchanged. At the other end of its travel, the damping of the TPS signal is great – no matter what happens with throttle position, the signal to the ECU varies only very slowly. Setting the pot at ‘in-between’ values gives variable amounts of signal damping.

    After some experimentation, a 10 kilo-ohm pot and a 100uF capacitor were used. The cap must be rated for car voltage and note it is polarised – negative lead to ground.

    Here is a trace of voltage over time, recorded on a Fluke Scopemeter. The bottom trace is the raw signal from the TPS; the top trace is what the ECU sees after the filtering. As can be seen, as far as the ECU is concerned, all throttle movements become smooth and gradual.

    Hey – instant ultra smooth driver!

    Highlighted here is an example of the throttle being abruptly closed. The bottom trace shows the voltage out of the TPS plunging vertically, while the top trace shows that after the filter, it takes about half a second to drop to the same level.

    Here the throttle has been rapidly moved up and down. The top trace shows how this has been smoothed.

    (You might think from this data that the driving response is made awful – but it isn’t. Remember that the throttle blade opens and shuts as normal – it’s mechanically linked to the accelerator pedal. It’s just the car’s ECU that ‘thinks’ the throttle is opening and closing more slowly than it actually is.)

    I set up the circuit and did some on-road testing.

    With the damping set to a high level, the car’s driving behaviour was clearly changed. Despite the fact that the throttle blade is manually controlled, high signal damping resulted in revs ‘hanging on’ when the throttle was lifted. (Presumably the idle speed control air bypass was delaying the decrease in revs.) High damping also resulted in rather lethargic throttle response. However, the car drove far more smoothly, without the standard little jumps as the electric assist and regen braking occurred.

    With the damping set to a medium level, the car drove more smoothly that standard, with few downsides. In fact, if anything it felt like it had a larger engine. Plonk the throttle down and the electric assist came on smoothly and progressively, while lift off abruptly and there was a pause before regen braking again came on smoothly. This latter point – a slightly delayed regen braking on throttle lift – is quite advantageous as in normal traffic, it’s common to lift your foot and then want to almost immediately re-apply it. With the delay in regen braking, this sequence is smoother and likely to be more economical on fuel (as the car doesn’t slow as quickly – better to retain the forward motion rather than charge the HV battery).

    No ECU fault codes were logged with the filter in place.

    With medium TPS damping, the car certainly drove better than standard. But would it also stay longer in lean cruise mode – the whole point of the modification?

    The short answer is: yes it did.

    Testing the fuel economy of cars like the Insight and the Prius is very difficult to do accurately. My test is confined to open road fuel economy and is conducted on the mostly flat freeway between the Gold Coast and Brisbane in Queensland, Australia.

    In the past I have driven from Oxenford to Brisbane Airport every day for about four weeks, recording the fuel consumption each day (see My Commute). Typically I got a displayed economy of 2.9 litres/100km for the one-way trip. The best ever was 2.8, the worst about 3.0 litres/100km.

    On a similar but longer drive, (eg to the north of Brisbane, some urban running and then return to Oxenford) I normally achieve 3.1 – 3.2 litres/100km.

    Note that these drives are always completed running at the speed of the other traffic (110 on one section of freeway, 100 on another, and then 80 – 90 for some sections of congestion, then often a section of heavy congestion at 0 – 20 km/h.) The baseline is with the air conditioner not running. No special driving techniques are used – no pulse and glide, no in-neutral rolling, etc.

    In short, this is a normal, real world drive - not one where the car is dawdling along in the slow lane, or holding up other traffic through leisurely acceleration.

    Note that my Honda Insight has been fitted with a slightly revised intake system – see We Have a Record!.

    With the TPS smoothing circuit in place and MoTeC air/fuel ratio meter continuously displaying mixture strength, I hit the freeway.

    Immediately noticeable was that the car stayed in lean cruise mode far more of the time – slight rises in the road needing more throttle would previously have seen the car drop out of lean cruise, but now the car stayed at 25:1. Any larger movement of throttle (eg when swapping lanes to pass a truck) saw the mixtures return to stoichiometric, but in general the mixtures stayed in lean cruise - except when periodically doing the short-term rich mixture NOx burn-off.

    As I said – it’s hard to accurately measure fuel economy when so little fuel is being used. However, in addition to the trip computer figure the best ever recorded, the conditions of the drive (winds and traffic congestion) were, if anything, conducive to poorer fuel economy than usual, not the best ever!
    Driving on the Throttle?

    So with the filtering in place, what happens to throttle response when you want to corner hard, adjusting the cornering attitude with the throttle? Interestingly, there’s not a lot of difference.

    Three things come into play here – firstly, the throttle blade movement itself is unchanged. Secondly, unless you’re a racing driver, it’s unlikely that you’ll be ‘steering’ with tiny movement of the throttle. Instead it’s more likely that you’ll be using large movements. Finally, the Insight is a car with little power, so to change the cornering attitude you tend to get heavily off the throttle or heavily onto the throttle.

    In a more powerful car – and perhaps with a better driver – these aspects might not apply.

    Other Cars?

    Would such a TPS filter work effectively on other cars?


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  2. #2
    Lifetime Member Garry's Avatar
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    Jun 2006


    wow ...
    Garry Glendown * '17 Fifty 2SS MT * 99 Firehawk Convertible (for sale)

  3. #3
    Lifetime Member blindawg's Avatar
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    Sep 2010


    wouldnt all the extra air without the fuel to be mixed with it in that short split second that the smoothing occurs do damage? or is it so small that nothing is effected?
    Isnt this what the DBW does? i know this is an old post.


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