Answer me this please (genuine question):
ECUs idle an engine at a specific RPM - 650RPM or 700RPM.
When the RPMs drop below 650RPM, more mixture is injected to bring idle up to that RPM (as to move the engine at 650RPM, when it's running at 500RPM, more fuel is a necessity).
If you disable fuel and spark to two cylinders, the power being produced is less, the cylinders not firing (even dynamically), are a drag on the entire engine (the weight of the piston is still being moved).
When you move that piston, with less fuel being injected, the RPM will drop as there is an increase in drag on the engine (the piston that is not firing, even if that piston is variable).
Doesn't that simply mean that because the engine has to move more parasitic weight, that the cylinders that are firing, need more fuel to maintain the same Idle RPM (as without that extra injection, the RPM would otherwise fall)?
Limp Home Mode is a 'feature' of some cars where the ECU enters a "limp home" mode, where the engine speed won't exceed a certain threshold because of a failure with something. I think in that situation all cylinders do still fire, but the ECU won't pass a certain rev range.
Electronic throttle control - Wikipedia, the free encyclopedia
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All cars having a TPS have what is known as a 'limp-home-mode'. When the car goes into the limp-home-mode it is because the accelerator and engine control computer and the throttle are not talking to each other in a way that they can understand. The engine control computer shuts down the signal to the throttle position motor and a set of springs in the throttle set it to a fast idle, fast enough to get the transmission in gear but not so fast that driving may be dangerous.
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So it still runs as it should in terms of spark and fuel, but it limits the revs so that you can't go so fast and you end up as the name suggests, 'limp[ing] home'.
This won't limit the RPMs - because that's controlled by the ECU in FI cars, and so my question I think is the most relevant - if you reduce the number of cylinders, the RPMs will drop as well, and to bring the RPMs back up, more fuel must be injected into the remaining cylinders, and that applies regardless if the affected cylinder is variable or fixed.
Happy to be corrected - I might be (and have been) wrong.