What is the difference between cold start and hot start?
2 Answers
Cold start and hot start difference: 1. Fuel injection quantity: (1) Cold start: Enriched fuel injection. For gasoline engines, the gasoline entering the cylinder must fully evaporate and mix with air to form a sufficiently concentrated mixture before it can be ignited by the spark plug. At low temperatures, gasoline's volatility is greatly reduced, making it difficult for the engine to start. Therefore, during cold starts, fuel injection is enriched to allow more gasoline to participate in evaporation, forming a sufficiently concentrated mixture. (2) Hot start: Normal fuel injection. When the engine is hot, high temperatures are present everywhere inside it. Even if the ambient temperature is low, the intake manifold and cylinder still maintain high temperatures. At this time, the fuel injection quantity returns to normal during engine start. 2. Engine speed: (1) Cold start: The three-way catalytic converter and oxygen sensor cannot enter the working state, and with enriched fuel injection, the exhaust gas quality deteriorates. To meet cold start emission standards, manufacturers increase the idle speed during the cold start phase. At this time, the throttle opening increases, allowing more air intake, which can improve combustion conditions and enhance exhaust gas quality. (2) Hot start: The engine has reached normal operating temperature, the three-way catalytic converter's conversion efficiency is high, the oxygen sensor signal is normal, and the air-fuel ratio control is precise, so the engine starts with normal idle speed.
When I first started driving, I didn't understand the difference between cold starts and warm starts, but I later summarized the key points from experience. A cold start refers to starting the vehicle after it has been parked for a long time and completely cooled down, such as when you start the car in the morning to go to work—the engine is cold and needs to burn more fuel to warm up. The temperature rises slowly, the startup takes longer, and it may increase engine wear. The sound is also louder, and the car feels a bit sluggish, especially in northern winters. If you don't warm it up before driving, the power will be insufficient. I usually let it warm up for a minute or two to protect the engine. A warm start, on the other hand, is when you restart the car shortly after stopping, like after shopping—the engine is still warm, starts quickly and smoothly, consumes less fuel, and responds almost instantly. It's more suitable for daily short trips and reduces hassle. The biggest differences are startup efficiency and wear level—cold starts are more troublesome, while warm starts are much more convenient. I also found that frequent cold starts can shorten battery life, so I recommend minimizing multiple cold starts in winter by planning trips more efficiently.
