What is a major drawback of SI engines?

The major drawback of spark ignition (SI) engines is indeed related to compression pressures. SI engines typically operate at lower compression ratios compared to diesel engines, which allows for lower combustion temperatures and pressures. This design limits the engines' ability to extract maximum power and efficiency, as higher compression ratios can lead to improved thermal efficiency. However, high compression pressure can lead to knock or pre-ignition in SI engines, which can be detrimental to engine performance and reliability, necessitating certain fuels to prevent this issue.

In contrast, the other characteristics associated with SI engines, such as high fuel efficiency, low horsepower, or simple design, do not accurately reflect major drawbacks. While they can exhibit various levels of horsepower and some simplicity in design, these traits do not significantly hinder their performance or application like the issue with compression pressure does. Consequently, the impact of compression pressures on the efficiency and performance of SI engines marks it as the primary concern when evaluating their drawbacks.