A cold start is one of the most traumatic things that can happen to an engine. For the first few minutes that the engine is running, it is virtually blind, has poor circulation and is waterlogged. The goal of the engine module is to get the engine up to temperature quickly so emissions can be controlled. The oil in the crankcase plays a critical role in this process.
Air and Fuel
When the engine is started, it is running at its richest to make combustion possible.
During the first few minutes of a cold start, the oxygen sensor isn’t yet active and the engine is making an “educated guess” as to the air/fuel ratio using the coolant and air intake temperatures.
During some cold starts, the air-fuel ratio may drop to 9:1 or lower. The fuel tends to stay in droplets and not vaporize. The large droplets may not fully combust and the fuel could wash the engine oil from the sides of the cylinder and cause wear. The fuel can also get into the oil and dilute the oil in the crankcase. It gets worse because with a cold engine the clearances between the pistons, rings and cylinders are greater.
A rich condition means less oxygen, while a lean condition means excessive oxygen. A rich condition would mean the PCM wants to lean out the fuel mixture, but it can’t do this when the engine is cold. Less oxygen in the exhaust stream means there is not enough to light the converters so emissions can’t be controlled. This is also the harshest time for the converter when the most damage is done to the catalyst beds.
The pump cell controls the oxygen concentration of the sensor by adding or subtracting oxygen to the diffusion gap. The input to the electronic circuit modifies the oxygen concentration by changing the polarity of the current flow in the pump cell. The changing polarity of the input and trim current flow causes the control circuit to send a rich or lean signal to the engine control module.
It is not until the oxygen sensor’s pump cell reaches around 600° F before it starts reading the oxygen content in the exhaust stream. Once the oxygen sensors are up and running, then it can bring the air/fuel ratios under control.
In the past 25 years, oxygen sensors have improved their ability to warm up in less time using better pump cell designs, ceramics and heaters. They have also helped to reduce warm-up times by getting the oil and coolant up to temperature sooner. This is achieved by running leaner without damaging the engine due to pre-ignition or excessive fuel bypassing the piston’s rings and entering into the crankcase.
Modern engine oil does a trick during startup. When the oil is cold, it flows five to six times better than when it is hot. When cold, it is thinner and can be pumped easily to the upper parts of the engine to lubricate the bearings of the camshafts. Also, thinner oil transfers heat quicker because it moves faster in the engine and flows in areas of the engine that warm up quickly when compared to the coolant in the water jacket.
As the oil warms up to operating temperatures, it becomes thicker or more viscous and can protect the engine better than can a thinner oil. It also can absorb more heat.
This remarkable task is done with some extraordinary chemistry in the oil base stock and the additive package. The ingredients that make multi-viscosity possible have a limited life inside the engine and break down due to time, temperature and mechanical forces. If the oil is not changed regularly, it might do the opposite and become thick at startup and thinner when the engine is up to operating temperature.
Air contains water, the amount of which in the atmosphere can change due to barometric pressure and temperature. The water can drop out of the air if it comes in contact with a surface in the engine that is at the dew point or lower. This condensation or liquid water can get into the oil. The water molecules can even combine with combustion byproducts to form compounds like sulfuric acid that can damage the engine.
The condensation is boiled off during normal operation because water has a boiling point of 212° F and the oil has an operating temperature between 220 and 225° F. Water can build up if the engine never fully warms up because of short trips or a blocked PCV system.
Recently, Mercedes-Benz issued a TSB where the instrument cluster displayed a warning that the oil level was too high in some vehicles. The common denominator was owners who used their vehicles for only short trips. The water built up enough in the crankcase that it set off the oil level sensor. The cure for the problem was to reduce the oil change interval.
Oil that does not regularly reach operating temperatures can turn to sludge that can build up in the engine.