Using analogies is always dangerous because they're never going to be a perfect fit. But the difference between your engine analogy and the federal reserve's actions is that anemic economic growth and high unemployment is not a side effect. In your engine analogy, adding more gas and air into the engine increases the amount combustion. It's the combustion that increases speed, so in a sense increase combustion is a goal. Released heat is just a by product of the reaction. In the economy, the amount of economic activity and the amount of available labor are what drive prices, so these things are the combustion in the piston, not a byproduct.