It seems like every time we buy a new car the fuel efficiency does not significantly increase. It has been 30 years! Why can't scientists make a breakthrough?! #HeatEngines #Explained #ChemE (1/31)
Today’s passenger cars have an average gas mileage of about 31 MPG while cars built 30 years ago still get 27 MPG.
Yup this is only a 15% increase in 30 years. But why? (2/31)
Yup this is only a 15% increase in 30 years. But why? (2/31)
In this thread we will explain why engines have struggled to advance in their efficiency using the Second Law of Thermodynamics. (3/31) 
Now I know many of you may be saying, “How will I ever understand the Laws of Thermodynamics, I’m not a rocket scientist.” Well fear not, because by the end of these tweets you will be able to say you know rocket science (Or at least why an engine cannot be 100% efficient). 4/31
Cars are powered by internal combustion engines. Sounds complicated, yuck! But these engines can be more generally classified as heat engines. (5/31)
A heat engine relies on heat transfer to do work. Heat transfer occurs spontaneously from a hot reservoir (Qh) to a cold one (Qc). A heat engine then uses part of the heat transfer to do work (W). (6/31)
You may be thinking this heat engine is way too simple to power my car but actually it is. Let’s take a look … (7/31)
Here is a car engine. The piston moves up and down which powers the car. This process can be represented by 4 steps … (8/31)
Fuel intake
Fuel combustion
Power generation
Expel exhaust (9/31)
Fuel combustion
Power generation
Expel exhaust (9/31)
It’s easy! We just solved the energy crisis. We just need to convert all heat (Qh) into work (W). (10/31)
ALERT: Second Law Police … This is impossible!!! (11/31)
The Second Law states a 100% efficient heat engine will NEVER HAPPEN!! Think of this like trying to get change for a dollar but only receiving two quarters. (12/31)
Now let’s look into why we will never get a 100% efficient car engine … (13/31)
But first let’s define efficiency. Efficiency is easily defined by the work out (W) vs energy in (Qh). (14/31)
Or with some algebraic manipulation, where W = Qh - Qc … (15/31)
Furthermore, Nicolas Léonard Sadi Carnot simplified this efficiency equation even further. He said for a perfect engine the Qc/Qh ratio equals the ratio of absolute temperatures of the thermal reservoirs. (16/31)
For cars the energy in would be considered the intake of the fuel source. So for our purposes we’ll use the combustion of gasoline. (17/31)
The gas enters the engine cylinder and is ignited, causing the inside of the piston to increase in temperature to about 2500 degrees Celsius. (18/31)
Guess what that means? Yes, we have the Qh part of our heat engine. (19/31)
But we know that we also need a cold reservoir for this engine to do work. Any guesses on what that will be? (20/31)
If you guessed the outside air then you would be correct. Yes, even at noon on the hottest summer, so let’s say 38 degrees C. (21/31)
Now let’s do the math!! If we have a perfect engine with a Th of 2500 oC and a Tc at 38 oC we get an efficiency of less than 100%, specifically about 89%. Remember we must use absolute temperatures to find efficiency! (22/31)
So to get 100% efficiency we would need a HUGE temperature difference between thermal reservoirs. But is this really realistic?! (23/31)
Can we have car engines at temperatures as hot as the sun and as cold as absolute zero? No, this would just be crazy! (24/31)
However, a temperature driving force is not the only effect on efficiency. Friction also plays a large role in loss of engine efficiency. (25/31)
Even in a very well oiled engine, there still exists a pretty significant amount of friction. Any time any material touches another material there will be friction, this even goes for anything traveling in air. (26/31)
All that friction and temperature difference really start to add up. Now you see why it seems like you’re always filling up your tank. (27/31)
But what if we did have impossibly frictionless parts and an infinite temperature difference could we then have the perfect cars that we so long for? (28/31)
Well once again the second law of thermodynamics comes into play. (29/31)
Since we are effectively converting heat into work, we will never get a complete conversion which is why car engines are only about 35% efficient. Such is the way of our world. (30/31)
So the next time you find yourself paying $40 to fill up your car, don’t be mad at the fuel source, be mad at the physics that govern our life. (31/31)
