Cars Add a Lot of Heat to Our Cities—It's Time to Ban Them

We calculate it in patio heater equivalents.

Manhattan Street is full of cars

fotog / Getty Images

We worry about the carbon dioxide emissions from cars, but in times like these, should we also worry about the heat? According to the U.S. Department of Energy, 68% to 72% of the energy from the fossil fuels you put in the tank is wasted as heat through the radiator and the exhaust. We are not only heating the planet indirectly through the car's carbon dioxide (CO2) emissions but directly by burning fuel. This raises questions: How much heat are we adding to our cities every day, and how can we make it feel relevant?

Energy lost from car

Office of Energy Efficiency and Renewable Energy

The first and most important fact is the number stated by the Department of Energy is low; that is just the direct thermal radiation. The thermodynamic reality is that essentially 100% of the gasoline is converted to heat.

As mechanical engineer Steve Blumenkranz noted, "The heat dissipated by a car by all loss mechanisms combined is equal to the heating value of the gasoline burned by its engine. All of the energy in the fuel is eventually dissipated as heat." Another commenter in Quora confirmed that  "everything from that is liberated from fuel burning is dissipated as heat."

As for the small fraction of energy converted into forward motion, "This also eventually turns into frictional heat." Looking that the Department of Energy drawing, it's pretty clear everything turns into heat—even the power to the wheels.

Patio Heaters

Jason_khaw / Getty Images

How much heat and what would be a relevant comparison that people would understand? How about patio heaters, which are reviled by environmentalists and described by a British politician as "an absurd invention" because "it is ludicrous that people are trying to heat the open air, as well as being irresponsible in the light of the climate change challenge we face." A large patio heater has an output of 50,000 BTU/hour—that's 14.6 kilowatts. This will be our new unit for unwanted heat: the patio heater equivalent (PHE).

Another Quora contributor, Matthias Holl in Munich, noted: "All the energy that is released by burning up fuel in the car is eventually converted into heat - either directly via the heat of the motor and heated exhaust gasses (the biggest chunk) or indirectly by heating up the air through air friction while driving, heating up the tires, sound waves converted into heat etc." He did some math and calculated the average car's consumption of gasoline per hour (7 liters) and its calorific value of 60 kilowatts, so driving your average car around the city is the equivalent of four patio heaters running full blast.

Since commenters on Quora are not necessarily reliable sources, I did the math myself here, trying to figure out how many patio heaters are unleashed each day on Manhattan.

Note the average fuel economy is for cars, not light trucks, and they are doing city driving, so the number of gallons of gas consumed in a day in Manhattan may well be considerably higher.

Normally our real concern is the 8.887 kilograms of CO2 emitted by burning a gallon of gasoline, or the 13,510 metric tons of CO2 per day just from cars driving around Manhattan. But when we are having a heat wave, those 157,336 patio heater equivalents cranking away are going to make a significant contribution. Perhaps when the heat is on, we should be turning the cars off.

Given the amount of asphalt they need for moving and parking—all of which contribute to the heat island effect, and much of which could be used to plant trees—perhaps we should just get rid of most of the cars if we are serious about cooling our cities and reducing both heat and carbon emissions. This, of course, heavily relies on ensuring cities have public transport infrastructure. There's much work that needs to be done and the time to start it was yesterday.