What is the difference between cold start and hot start of a car?

[DeMaximus]

Car cold start and hot start are different as follows: 1. Cold start: It means starting the engine when the water temperature is low; cold start is more harmful to the car, and more than 80% of engine wear occurs during the cold start phase. During a cold start, the gaps between various parts inside the engine are relatively large, and it takes time to warm up and reach the specified gaps through thermal expansion, because if the gaps are too large, the oil film cannot cover and lubricate. During a cold start, the internal temperature of the engine is too low, making it difficult for the air-fuel mixture to vaporize and burn. 2. Hot start: It means starting the engine when the water temperature is high, the only difference being the temperature of the cooling water between the two.

[ChristopherLynn]

The difference between cold start and hot start, as I often notice while driving: A cold start is when the engine is started after it has cooled down, such as in the morning or after parking for several hours in winter. At this time, the engine sound is louder, there's a bit of shaking, and fuel consumption is slightly higher because the oil is too thick, causing more friction between parts, and the car's computer needs to inject more fuel for lubrication. What about a hot start? For example, restarting after parking for five minutes to buy groceries, the car starts instantly, smoothly and silently, with normal fuel consumption. I've experienced several times that during a cold start, the battery struggles more, while during a hot start, the battery works effortlessly. The key lies in the temperature difference: cold engine parts contract tightly, making starting harder; warm engine parts are heated and lubricated better. It's recommended to warm up the car for a minute or two before driving in cold weather to protect the engine and reduce wear.

[LaJanelle]

I'm quite concerned about startup issues when using my car for daily commuting. Cold starts, like in the morning after the car has been parked overnight, involve loud engine noise, unstable RPM, and an initially sluggish feel. Hot starts are simpler – restarting after being parked for half an hour results in instant ignition and smooth operation, quiet and effortless. The main differences lie in temperature and internal conditions: during cold starts, the engine is cool, engine oil is thick like syrup, creating more resistance, and the ECU adjusts to a fuel-rich mode; during hot starts, the oil flows freely with normal air-fuel ratio. This affects fuel consumption – cold starts consume an average of 0.5 liters more fuel. It also impacts battery life: cold starts drain more power, while hot starts are gentler on the battery. I've tried turning off the engine during short stops to save fuel, and hot starts prove truly efficient.

[JasonFitz]

The difference lies in the physical conditions during cold and hot starts. When the engine is cold, compression pressure is lower, lubricant is more viscous, the starter motor has to work harder to turn the engine over, and the battery drains faster. During a hot start, the engine is warm, oil flows better, and the engine starts smoothly almost instantly. The air-fuel ratio is richer during a cold start, resulting in slight smoke emissions; during a hot start, the ratio is more balanced, making it more environmentally friendly. The reason is simple: thermal expansion and contraction of metals affects clearances. This impacts wear rates: cold starts accelerate component aging, while hot starts are gentler. A common example is that a cold engine shakes, while a warm engine runs smoothly—the contrast is obvious.

[LeFinley]

Focusing on environmental impact and efficiency, I've observed that cold starts emit more pollutants, such as unburned hydrocarbons; while hot starts produce cleaner emissions as the catalyst reaches optimal temperature. Cold starts are less efficient, leading to more fuel waste, whereas hot starts are highly efficient with almost no additional consumption. This stems from temperature differences: cold engines experience higher internal friction, requiring more fuel to compensate, while warm engines operate smoothly with reduced resistance. Minimizing cold starts—for example, by combining short trips—can benefit emission reduction and cost savings. The advantages of hot starts align well with sustainable driving practices.