Comparative　experiments　of　oil　and　water-cooling　were　performed　on　a　4-cylinder　automotive　gasoline　engine　and　a　single-cylinder　direct　injection　Diesel　engine.　Measurements　were　made　to　investigate　the　variation　of　fuel　consumption,　combustor　wall　temperature　and　engine　emissions　(HC,　CO,　NO　x　and　smoke)　with　two　cooling　media　at　steady-state　conditions.　Significant　improvement　of　fuel　economy　was　found　mainly　at　partial　load　conditions　with　oil-cooling　in　comparison　with　the　baseline　water-cooling　both　for　the　two　engines.　The　experimental　results　also　showed　general　trend　of　reduction　in　engine　emissions　using　oil　as　the　coolant.　Measurements　of　wall　temperature　demonstrated　that　oil-cooling　resulted　in　considerable　increase　of　the　combustor　wall　temperature　and　reduce　of　warm-up　period　in　starting　process.　For　automotive　gasoline　engine,　road　tests　indicated　the　same　trend　of　fuel　economy　improvement　with　oil-cooling.　The＇　performance　of　the　automotive　oil-cooled　engine　was　further　improved　by　internal　cooling　with　water　or　methanol　injection.