Why our Elise went into limp mode

Lotus read about our Elise trundling to a halt at Spa; comes back with the reason why

So if you were reading the blog from yesterday’s track day at Spa (we’re still there with Renaultsport!) you’ll have seen we had an issue with our Elise going into limp mode half way round the circuit. Lotus read it and got back to us pretty damned quickly with a definitive answer as to what happened. And it’s rather interesting.

The problem occurred left-foot braking into Pouhon at Spa, the entry being a downhill left-hander in fourth gear at around 95mph. Feeding the power back in, well, there wasn’t any. An orange spanner symbol appeared in the dash and it was clear the car was in limp mode. Cue Lotus, rightly, bricking it about a fresh wave of L.O.T.U.S. sniggering.

When it did it again later in the day in exactly the same circumstances the obvious conclusion was that there was something going on in the ECU. A Lotus spokesperson came back the following, and very detailed, explanation.

Stop trying to show off, basically

“What you experienced to put the car in limp-home mode is what we refer to as ‘BTO’ (Brake Throttle Override) which is a feature that helps to address a UA (Unintended Acceleration) in trapped accelerator pedal situations. As reported in the press and to NHTSA, most vehicle manufacturers, including Lotus, have already adopted brake-throttle override or will be incorporating it into all vehicle designs over the next few model years.” Not just those equipped with Toyota engines…

It’ll be a familiar situation to anyone who’s tried to left-foot brake a VW group car equipped with DSG but the detail of the Lotus response makes interesting reading. “In the case of the technology employed on the Elise, brake-throttle override is an electronic function of the engine control system. Generally, it works by continuously checking the position of the brake and accelerator pedals and by recognising when an acceleration command through the accelerator pedal is in conflict with a concurrent application of the brake pedal. If the BTO system identifies that a pedal conflict exists, it invokes the override function which causes the engine control system to ignore or reduce the commanded throttle input, thus allowing the vehicle to stop in a normal fashion. Regardless of the specific means used, BTO intervention quickly reduces or eliminates the unintended vehicle propulsion.”

Obviously this doesn’t happen if you brake with your right foot but you’d hope that a Lotus would allow you to play around with trail or left-foot braking and according to the engineers as long as you’re on the brake before the accelerator you can. Apparently so long as the ECU sees zero per cent throttle input when the brake is pressed all is fine.

And as we found out, if you are clod-footed enough to trigger a BTO situation the solution is that IT fall-back of turning it off and back on again. And stop trying to show off about how clever you (think you) are with your on-track footwork.

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Hatchoo

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