Monday 14th April 2014

Why our Elise went into limp mode

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

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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.

[Hatchoo]

Hmmm...whilst this system might permit left foot braking...it hardly encourages it. Not really in the spirit of the car.

[Dixy]

Am I reading that correctly, you will no longer be able to heal and toe ?

[Ved]

A can't imagine fat footed Americans buying an Elise anyway. Or am I just sceptical about UA for the vast majority.

Edited by Ved on Monday 14th April 14:51

[Matt UK]

Could it not be set up to be turned off manually when on track?

[otolith]

Dixy said:

Am I reading that correctly, you will no longer be able to heal and toe ?

No - it's happy as long as you are off the throttle before you get onto the brake, so H&T will be fine. Left foot braking is the issue.

Nannying bks to avoid being sued by incompetent geriatric Americans who couldn't get in or out of an Elise in the first place.

[TobyLaRohne]

otolith said:

Dixy said:

Am I reading that correctly, you will no longer be able to heal and toe ?

No - it's happy as long as you are off the throttle before you get onto the brake, so H&T will be fine. Left foot braking is the issue.

Nannying bks to avoid being sued by incompetent geriatric Americans who couldn't get in or out of an Elise in the first place.

Actually to me it makes some form of sense, if the car has a jammed throttle (you read about these cases mostly in the states) you'd be pretty happy to be able to push the brake and the car cuts the power for you. In a panic situation not everybody thinks to hit the clutch or knock a car into neutral, and that is why they end up either dead or very hurt and sue the company.

I'm not sure why it would need to put the car into limp mode though, the VW system works well enough by temporarily over-riding the accelerator and cutting power if you ride both pedals.

[mrdemon]

"Obviously this doesn’t happen if you brake with your right foot "

lol as it knows which foot you are using now !!!!

[TTmonkey]

mrdemon said:

"Obviously this doesn’t happen if you brake with your right foot "

lol as it knows which foot you are using now !!!!

Whoosh.

[TobyLaRohne]

mrdemon said:

"Obviously this doesn’t happen if you brake with your right foot "

lol as it knows which foot you are using now !!!!

well it would if you could brake with your right foot and accelerate with your left...give it a go next time you're in traffic.

[AyBee]

So who can't drive then?

Understand that this would be useful for day-to-day driving but a simple switch to turn it off ("TRACK") would probably be useful

[motoloco]

Ved said:

A can't imagine fat footed Americans buying an Elise anyway. Or am I just skeptical about UA for the vast majority.

Sorry, but can't help being pedantic. It should be sceptical; Only Americans (fat-footed or otherwise) write skeptical.

I know, I know....

[Snowboy]

TTmonkey said:

mrdemon said:

"Obviously this doesn’t happen if you brake with your right foot "

lol as it knows which foot you are using now !!!!

Whoosh.

I regularly break with my right foot while having my left on the gas.
It's the Riverdance driving style.

[Robert Elise]

h&t is when gear is in neutral, so i hope this will never be affected?
But sports cars should have an override.

[Ved]

motoloco said:

Ved said:

A can't imagine fat footed Americans buying an Elise anyway. Or am I just sceptical about UA for the vast majority.

Sorry, but can't help being pedantic. It should be sceptical; Only Americans (fat-footed or otherwise) write skeptical.

I know, I know....

Spell checker jumped it. Corrected. Now, back in your box

[RoverP6B]

Surely a track-focussed car like an Elise should have an override switch for this sort of thing, as well as (possibly) a line-lock valve to reduce the power rear brakes to neutralise understeer/enable big smoky burnouts... I can understand it for road use, but it does strike me as being something that should go off with the traction control (assuming Elises have that?).

[otolith]

TobyLaRohne said:

Actually to me it makes some form of sense, if the car has a jammed throttle (you read about these cases mostly in the states) you'd be pretty happy to be able to push the brake and the car cuts the power for you. In a panic situation not everybody thinks to hit the clutch or knock a car into neutral, and that is why they end up either dead or very hurt and sue the company.

In a Zafira or a Scenic, sure.

[RacerMike]

Not really understanding the need to put the car in to limp home. It's somewhat overkill on a car that will drive on track. A better strategy would be to reduce engine power after a time period (say around 1s) of both the TPS reporting greater than 10% and the brake system reporting greater than 10bar. Once the conditions are removed (i.e. once both throttle and brake are removed) have another timer upon which full power is restored. Then there's no need for an ignition reset. This is exactly how VAG have achieved the fail safe for the last 15 years!

[vz-r_dave]

otolith said:

No - it's happy as long as you are off the throttle before you get onto the brake, so H&T will be fine. Left foot braking is the issue.

Nannying bks to avoid being sued by incompetent geriatric Americans who couldn't get in or out of an Elise in the first place.

Yeah because the huge UK market should govern decisions car manufactures make..... pfff I won't bother looking at the numbers but I am confident Lotus has a larger market in the US or at least pottential market anyway. The ignorance on PH astounds me.

[Snowboy]

This happened to me the other day in my Land Rover when I was wearing flip flops.
But even then it just flashed a red light rather than go into limp mode.

[otolith]

vz-r_dave said:

otolith said:

No - it's happy as long as you are off the throttle before you get onto the brake, so H&T will be fine. Left foot braking is the issue.

Nannying bks to avoid being sued by incompetent geriatric Americans who couldn't get in or out of an Elise in the first place.

Yeah because the huge UK market should govern decisions car manufactures make..... pfff I won't bother looking at the numbers but I am confident Lotus has a larger market in the US or at least pottential market anyway. The ignorance on PH astounds me.

I would imagine that the majority of American Lotus buyers are not incompetent geriatrics and would dislike the feature as much as anyone else.