How does ambient temperature affect engine efficiency and fuel consumption?

Question

I remember from my physics class that the ideal-gas Carnot engine is more efficient if the temperature difference between the heat source and the thermostat ("heat recipient") is larger (http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/carnot.html).

And I always thought that this MIGHT be applicable to real-life engines, so I have to admit, I did expect lower fuel consumption during winter. Alas, I have not yet experienced the money saving power of a winter's frost.

But maybe my measurements were flawed, and combustion engines are indeed more efficient if excess heat is radiated more easily?

PS: I know that a cold engine also means more viscous/thick oil. So I did notice a significant loss of efficiency (power, really) right after starting my motor. But I'm more interested in a stationary situation, when the engine is warm and ready.

Answer 1

tl;dr: Ambient air temperature should generally not interfere with engine efficiency or fuel consumption, but will affect overall power output.

Do not confuse efficiency with power output. These are two separate things. When your intake charge is more dense, you can throw more fuel at it and creates more power. (NOTE: The idea for the engine management system is to maintain a general 14.6:1 air to fuel ratio (also called stoichiometric or "stoic" for short). This is the so-called "perfect" mixture of air and fuel where all of the fuel is burned with no extra oxygen being left afterwards. Unfortunately, the stoic mixture is not usually obtained. This happens because of two problems which arise, both having to do with the amount of heat which is created during the combustion process. First, the hotter combustion can cause detonation. Second, above a combustion temperature of around 1700degF, nitrogen in the air which is brought into the engine (along with the oxygen -- air contains ~78% nitrogen and ~20% oxygen) and burns. This creates NO2 or Nitrogen Dioxide. This is a major air pollutant and was the primary cause of acid rain talked about in the 70's in California. It is also very bad for us to breath -- toxic in fact.)

The flip side of this is efficiency, which in the context of engines means getting more usable power from the same amount of fuel. Great strides have been made in the past couple of decades towards engine efficiency. One of the ways they have accomplished this is through turbo-charging. In simple terms, turbo-charging is a way of utilizing the heat energy otherwise discarded in exhaust process. The turbo is able to increase the air charge by using the pressure created from the exhaust gases, which allows the computer to throw more fuel at the intake charge, thus making more power. This could lead to a very large "other" discussion, so I'll leave it here. Needless to say, power is made more efficiently through this method than through normal aspiration and thus the engine can make more power with less fuel.

Another way to improve engine efficiency is to increase the compression ratio (CR) of the engine. A general rule of thumb for CR is, for every point of added CR your power output will raise by about 3%. If you are increasing power output without adding more fuel, this is increased efficiency.

A colder air charge going into the engine will be denser and contain more oxygen than it's warmer counterpart. You are still using more fuel to create more power, thus there is no added efficiency benefit.

While you suggested to not include a cold start-up, there is a reason you will not see better fuel consumption during this period. The reason is because the computer actually throws more fuel into the mix to provide for increased engine stability (help keep it running smoothly -- like a choke would on a carbureted engine) and to help the catalytic converter warm up quicker helping it reach peak efficiency faster.

Actually, combustion engines can be quite a bit more efficient if they can utilize the heat instead of radiating it. Remember that radiated heat is lost energy. If you can utilize the heat to produce more power or create the same power more efficiently, you're just better off all together.

What I'm talking about is a concept a guy by the name of Henry "Smokey" Yunick had mastered in the early 80's. He worked off of an idea which Ralph Johnson came up with in the early '50's while Ralph worked at GM. The idea of a hot air engine in which the air is heated to around 400degF and homogenized (blended very well) to a point where it would not have detonation. You can read the article, but the reason it is not around in vehicles today is two fold. First, they tried to make it into a bolt-on kit, but couldn't do so because it required upgraded parts for the pistons and rings, which is in effect not so much of a "bolt-on" kit and makes it a lot more expensive than the target prices they were shooting for. Secondly, Smokey unfortunately died some time back. Way too many of his secrets died with him as he kept the particulars in his head. This is truly sad, because he did some truly AWESOME work and had revolutionary inventions and ideas which died with him.

The hot air engine flies in the face of common thinking about cold air induction and your question. Common wisdom states that the colder the air going into the engine, the better the output. And this is basically true with (what we consider today) normal engines (Smokey's hot air engine being an outlier).

Answer 2

Lower "cold side" of the Carnot cycle leads to better theoretical efficiency, sure, but have you calculated how much? 10-20 K colder intake with the same 1000 K combustion temperature affect final efficiency by 1%. And that efficiency is 70% in any case, so you can guess that there are so many more parameters lowering the final efficiency to 25%, that the Carnot has little relevance in a car.

And in any case, the engine has to be re-engineered to use that lower "cold side" if you want the same combustion temperature, because if you just lower the intake air in a normal engine, you end up lowering by the same amount also the "hot side" temperature", reducing the efficiency gain even more.

In winter cars use more fuel because air is denser and you have to push it away, tires have greater friction and you have to push them harder, oil in transmission gets thicker and causes more losses, you use heating and the other features that use energy (that comes from the fuel of course, you have less to actually move the car), the fuel is different in the first place to enhance combustion in cold temperatures (this means is modified to burn better, but has less energy inside so you need more), and so on.

You can have up to 50% worse fuel economy in winter, in specific situations even 100% worse (that means, the fuel consumption doubles).

As example of causes of higher fuel consumption: https://www.fueleconomy.gov/feg/coldweather.shtml

Edit

By the way, my mistake: the maximum efficiency of 1-(T_low / T_high)) applies to Carnot, but the Otto engine has a different maximum efficiency, see https://physics.stackexchange.com/questions/168912/carnot-vs-otto

It also means that the engine is already way closer to the theoretical efficiency than what it's often though.

Answer 3

When taking an engine running on otto's thermodynamic cycle (the work gained for each cycle is the area inside the P(V) diagramm) against the same load and with optimal running temperature everywhere in its lubrification system : taking a colder air intake means that the mixture can be made leaner because most of the time, (less and less so as ECU computing power progresses) the mixture is kept on the rich side of the 14:1 stoechiometric burning reaction to lower chamber temperatures and avoid unwanted pre-detonation (knock). If the engine is turbo-charged the temperature between hot and cool side of the turbine will be greater and the intercoller(s) will extract more heat out of the compressed air. which implies efficiency gains on every parts of the operation. So the power gains looked after in motorsports by use of "cold air intakes", bigger more efficient intercoolers, turbo "blankets" to keep the hot side of the turbine hotter and the "cold" part cooler all those are also beneficial when looking for fuel efficiency. However in a vehicle moving through air, those thermodynamic gains can be quickly negated by the augmentation of aerodynamic drag if th air encountered happens to be denser ( which is not assured by temperature only but also by vapor content.