Caution: Math Forthcoming

The mathematics of aerodynamics and power can become quite intricate. Therefore, I will simplify things with a basic model that highlights the main factors influencing fuel consumption. While this model is adequate for illustrating fuel economy, the EIA’s Fuel Economy page provides more in-depth data on various elements that affect vehicle performance.

The Toyota Rav4 serves as an excellent example for direct comparisons, as it comes in ICE, hybrid, and electric variants. All models share the same chassis and similar engine specifications, making them ideal for such evaluations.

First, I calculated the energy required to propel a vehicle. The primary forces to counteract are rolling resistance (affected by road conditions, tire quality, and weight) and air resistance (dictated by the vehicle's size and shape). By providing power to the wheels that matches the combined rolling and air resistance, a vehicle can maintain a steady speed. This is crucial to understand, but for those interested in the math, the equation is as follows:

For those who prefer to skip the calculations, the key point is that air resistance escalates rapidly as speed increases. I can estimate the rolling resistance and drag based on the specifications of the Rav4 to determine the power required to keep an ICE cruising.

At highway speeds, where energy consumption peaks, air resistance is far more significant than weight. Consequently, a flag waving from a truck bed could negatively impact fuel efficiency more than a load of bricks.

Here’s how the three vehicle types compare:

The mass differences among the three trims are minimal. The EV variant is not the heaviest vehicle on the road; it weighs roughly the same as a base model Ford F-150.

The additional 750 lbs of the EV trim requires a negligible increase in power. This aligns with our everyday experiences; we typically don’t observe a significant drop in mileage with added passengers or cargo.

Understanding Latent Power

While cruising power is relatively similar across vehicle types, this does not fully explain why electrified cars are more efficient than ICEs. Gasoline contains approximately 33.7 kWh of energy per gallon, meaning that a Rav4 traveling at 45 MPH could theoretically achieve close to 200 MPG!

However, to round out the model, we must consider latent energy consumption.

A conventional gas engine loses over 60% of the chemical energy from gasoline as heat during combustion. Conversely, an EV can utilize nearly all of the energy supplied, although if that electricity comes from fossil fuels, the burning of methane to generate that electricity is only about 45% efficient.

In a gas vehicle, much of the remaining 40% of "usable" energy is not used for propulsion either. An engine idling still consumes fuel. This is known as latent power: energy consumed to keep the engine operational regardless of the vehicle's speed.

The latent systems of an ICE can consume up to 0.5 gallons of gasoline per hour. This translates to energy expenditure per distance: higher speeds tend to consume less latent energy per mile.

Many modern vehicles, including ICEs, now feature systems that shut off the engine at stoplights. While some drivers dislike this "feature," it significantly reduces latent waste. It’s also advisable not to "warm up" your car during winter months, as idling is incredibly inefficient in terms of MPG.

Latent energy consumption peaks at low speeds, while active consumption rises with speed. This creates an energy consumption curve for ICEs as follows:

With a 60% loss due to exhaust heat, my calculated efficiency for an ICE Rav4 appears as follows:

For comparison, here’s the EIA’s graph for a generic ICE. Their advanced modeling includes more factors affecting performance, making it more precise, but I believe my amateur analysis holds up fairly well.

Electrified vehicles experience significantly lower latent consumption. Hybrids optimize engine performance by turning off the engine at lower speeds, recharging the battery at moderate speeds, and combining both power sources during high-speed travel.

With the assistance of an electric motor, hybrids can utilize smaller engines that idle using less energy (around 0.15 gallons/hour) while still delivering ICE performance when necessary.

EVs, lacking any gas engine, utilize minimal latent energy, regardless of the energy requirements of the vehicle.

Overall, the combination of a battery and electric motor proves to be more efficient. The HV and EV powertrains excel in urban environments due to their low latent consumption. At highway speeds, however, there is little difference in fuel economy, as the extra weight diminishes the advantages of electric variants.

The Role of Regenerative Braking

Up to this point, I have assumed the vehicles are traveling on level ground at a constant speed. This does not reflect real-world conditions, where hills, stoplights, and sharp turns require acceleration and deceleration.

At first glance, one might think that the added weight of HEVs and EVs would hinder their performance in these scenarios, as acceleration power is directly related to mass. Surprisingly, HEVs and EVs perform exceptionally well.

It’s true that starting from a complete stop requires more energy for electric vehicles to reach city speeds due to their additional weight.

However, as these vehicles decelerate, they can recover some of their kinetic energy to recharge the batteries. The drivetrain operates in reverse: the wheels turn the generator (similar to a hand-cranked radio), instead of the battery powering the wheels.

Tesla claims that during a stop-start cycle, they can recapture up to 70% of their kinetic energy for reuse during acceleration. In contrast, ICEs waste all kinetic energy through braking as heat.

In my simulation, where cars go through five stop signs, accelerating from 0 to 45 MPH each time, the electrified vehicle with regenerative braking clearly outperforms others.

The effectiveness of regenerative braking in real-world scenarios largely depends on driving style and conditions. On highways, where constant speeds are maintained, the advantage diminishes. Regeneration is most effective during gradual deceleration, meaning drivers who brake harshly will gain less benefit. The EIA estimates that HEVs recover about 5-10% of energy through combined city/highway driving.

Additional Fuel-Saving Techniques

While HEVs and EVs generally outperform ICEs, various fuel-saving innovations could be implemented across all vehicle types but are often overlooked.

For instance, all vehicles can benefit from designs that reduce air resistance (like Tesla's flush door handles). EVs are also free from the need for an air intake grill and can adopt streamlined front-end designs.

Hybrids tend to feature transmissions with higher gear counts, allowing for improvements in fuel efficiency of 5-10% per additional gear (though returns diminish). This can raise initial costs and potentially reduce towing capacity. The Rav4 exemplifies this: it comes standard with an 8-speed transmission for the ICE, while HV trims utilize a more efficient Continuously Variable Transmission (CVT).

Weight: Just a Number

My vehicle fuel economy model confirms what we already suspect: electrified vehicles are indeed more efficient, despite their greater weight. When utilized correctly, mass has a negligible impact on fuel economy.

The true advantage of electrified vehicles lies in their efficiency at lower speeds. HEVs and EVs excel in stop-and-go traffic, where latent energy waste is significant and regenerative braking opportunities abound.

Drivers who transition to these vehicles often notice immediate financial savings. Collectively, we also reap environmental benefits from reduced emissions per mile.

For those currently transitioning to fully electric vehicles, it's important to recognize that EVs still contribute to carbon footprints, as much of the electricity is still generated from fossil fuels. The complete environmental benefits will materialize when our electricity sources are entirely renewable. For all nations committed to the Paris Agreement, this is a matter of when, not if. However, the timeline depends on our collective political actions.