Entropy generation during chemical reactions can cause significant irreversibility in chemical engines. These irreversibilities reduce the exergy of the fuel resource preventing potential work production. Understanding the cause of these irreversibilities and developing strategies for reducing them is critical for increasing engine efficiency. All chemical engines can be separated into two categories depending on how they manage the chemical reaction: restrained and unrestrained. A fuel cell with an electric motor is an example of a restrained engine design. Restrained engines have the potential to operate near the reversible limit, with no entropy generation from the chemical reaction. Combustion engines are unrestrained engines, which means the engine design has a built-in irreversibility due to the way it conducts the chemical reaction. This paper defines the requirements necessary to build restrained chemical engines, which helps to identify fundamental strategies for increasing efficiency in both engine designs as well as trade-offs between the two options. ► There are two natural limits for performing the chemical reaction in a chemical engine: restrained and unrestrained. ► The requirements necessary to build a restrained chemical engine are discussed in general terms. ► The criteria for improving the efficiency of restrained and unrestrained chemical engines are highlighted.