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Knock limited Engine

Understanding AFR

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I would like to know the definition of a knock limited engine.

Thanks for the help

It's basically when an engine maximum performance is limited by knock (detonation), which is a type of abnormal combustion. This type of abnormal combustion will damage pistons and ring lands, etc...

Engines that are not knock limited, you can put as much timing as you want, and you won't experience knock. So the maximum performance will be limited by something else, like the engine design for example.

I would say that is an engine where knock occurs prior to actual MBT point.

To clarify, the fual-air mix doesn't burn instantly, but over a small period of time and so the pressure buildup is progressive.

To get the best use of the expanding/increasing pressure gas we will ignite the mixture a little before TDC and trade of the increased resistance against the rising piston against the pressure pushing it down on the power stroke. The point where we get the most use out of the burning fuel-air is called the Mean Best Torque (or power, at that rpm, if you prefer).

With some engines we can advance the ignition past the point where we get the most use out of the fuel and torque will start to fall off, as the balance moves against MBT. These engines are regarded as not being 'knock limited', as we can advance past MBT.

However, with some engines, as we advance the ignition we will reach a point where it starts to detonate (or in some instaces pre-ignite) even though the MBT was still rising at that point and hadn't reached it's peak. These engines are called 'knock limited', because the 'knock' prevents optimal timing.

In some instances, using a higher fuel octane and/or water/methanol injection can allow the MBT to be reached.

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