...or, in EV terms, your mi/KWhr?
The answer is... It depends. And, it depends more if you're driving and EV.
We've been driving gasoline (or diesel) cars for a loooong time. We think about fuel economy in terms of mile per gallon. We kinda know that we get better mileage on the highway than driving around town. We've been told things to help us improve our MPG, like "avoid jackrabbit starts" and "avoid abrupt stops". The government has even gotten in on the game, mandating fleet average fuel economy numbers and publishing "EPA city and highway" fuel economy numbers, e.g. "18 city, 27 highway mpg".
Now, EVs show up and the whole game seems upside down. How do you even compare? One way is with MPGe, or MPG equivalent where you take the electrical energy and just convert, knowing there are 33.7 KW-hrs in a gallon of gas, if you burn it. This is a pretty fair comparison, actually. If you buy a gallon of gas, it costs about $3.00. To buy 33.7 KW-hrs at 12 cents per, it would cost $4.00.
I compared a Hyundai Ioniq 5 with a Hyundai Tuscon (non-hybrid) - similar sized cars - using spreadsheet calculator I built and some trip profiles I created. Here's the result.
If you'd prefer it in cost per mile...
What the heck is going on? Is it because of how the cars are propelled or because the fundamental differences in design make EVs move down the road more easily?
There are two parts to making a car go: Propulsion and Resistance. Propulsion is what pushes the car forward - the engine or motor and the drive train than connects that to the wheels and wheels and tires to the road. Resistance is what pushes back against the propulsion - primarily the friction/rolling resistance between the tires and the road and the aerodynamic drag - pushing through the air. Another part of resistance is inertia, that is pushing to accelerate the car up to speed.
On the surface, resistance wouldn't seem to have much impact. The EV is a bit heavier, because of the propulsion battery. Even replacing a heavy engine and transaxle, the EV comes in at 4700 pounds vs 3900 for the ICE.But, the EV is a bit more aerodynamic with a coefficient of drag of 0.29 vs 0.33 for the ICE. The reason here is the ICE needs to allow a lot of air directed under the hood and over the radiator to keep the engine cool. It is also harder to keep smooth airflow under the car having to deal with exhaust pipes and drive shafts.
Let's take a close look to see how these differences actually play out.
For my Ioniq 5 EV, it takes about 70 lbs of force to overcome rolling resistance. To make the car roll, you have to push it with 70 lbs force. To make it roll one mile, you need 70 x 5280 = 370,000 ft-lbs of energy.
At 30 mph, you need 5.7 HP. At 60 mph, you need double, 11.3 HP.
If there was no aerodynamic drag, you'd get a steady 7.5 mi/KW-hr. It wouldn't matter how fast you went. It takes the same energy to move each mile regardless of speed. Speed just tells you how fast you have to supply that energy.
The 0.21 KW-hr to 60 mph doesn't look like much compared the 3.5 it takes to move the car a mile at 60 mph, but driving involves lots of braking for stop signs, traffic lights, traffic, etc. Every time you slow down, you have to spend some energy to accelerate again and that energy is later destroyed as heat from braking.
The negative power is available for regenerative braking. A car without it burns it all off as heat - and has to worry about overheating the brakes or using "engine braking".
This shows dramatically the effect of regenerative braking.
This shows exactly what many of us are familiar with. Best mileage on the highway. Worst in local driving. Really fast highway driving starts lowering fuel economy. But, the overall effect is a rather flat curve. The difference between suburban driving and highway driving is minimal.
What range would that 77.4 KW-hr battery give me at various speeds?
This shows the problem the engineers have in estimating the range to display on the dashboard. They have no idea how fast you are going to go, they only know how fast you've been going. Using recent history to estimate is the best they can do. So, if you've been doing a lot of local driving at 40 mph, it might show 350 miles range for a full battery. If start going a steady 80 mph, you're actual range is only 190 or so.
Resistance
Let's start with the resistance. Here's the fancy equation for steady speed.
Where:
- cR = coefficient of rolling resistance
cD = drag coefficient
m = mass of vehicle
A = frontal surface area
g = acceleration of gravity
r = density of air
The first part is the rolling resistance. It's linear and goes up with the weight of the car. It also depends on what the wheel and road are made of. Cars tires on concrete have a CD of about 0.015.
https://www.omnicalculator.com/physics/rolling-resistance
How fast you can cover the mile depends on the power available. Power is the rate at which energy is used. If a person pushing on a car can produce about 1/4 HP, it would take 45 minutes to cover a mile (370,000 ft- lbs x (1 HP/33,000 ft lbs/min) x 1 person/0.25 HP)
I baked this into a spreadsheet and calculated for various speeds.
At 30 mph, you need 5.7 HP. At 60 mph, you need double, 11.3 HP.
If there was no aerodynamic drag, you'd get a steady 7.5 mi/KW-hr. It wouldn't matter how fast you went. It takes the same energy to move each mile regardless of speed. Speed just tells you how fast you have to supply that energy.
Now lets add in aerodynamic drag - the second part of the equation. It really is a drag. It goes up with the square of speed. Every time you double your speed, you quadruple your air resistance.
It depends on two factors- how aerodynamic the vehicle is and the frontal area of the vehicle. Shape determines the aerodynamics. A flat fronted truck isn't very aerodynamic as it plows it's way through the air, but a smoothly shaped car would be very aerodynamic as it slices it's way through. Size also matters. Two similarly shaped vehicles with different cross-sectional area would have different aerodynamic drag. The larger one would have proportionally more drag.
I plugged the numbers for my Ioniq 5 EV into my spread sheet. Here's the results.
At 30 mph, it takes 16.6 lbs force to move through the air. At 60 mph , it takes 66.2 lbs.
Including the speed you are going to the when you are producing that force, gives you this HP curve:
At 30 mph, it takes 1.3 HP. At 60 mph, it takes 10.6 HP. Speed kills energy efficiency.
Putting the two parts together, you get this:
Doing the same for the Tuscon and comparing, this is what you get.
Notice the Tuscon rolls a bit easier at low speeds because it's lighter, but a bit worse at high speeds because it's less aerodynamic.
The takeaway here is the differences between the cars in terms of resistance are very slight.
Some more resistance
There are a couple of other ways the car "pushes back."
One is inertia. If you apply a force to an mass, it will accelerate. It takes energy to go from a stop to speed or from a lower speed to a higher speed. A mass moving at speed gains kinetic energy as its speed increases. That energy comes from the propulsion system. The equations for this is are
F = ma
where:
F is the net force applied
m is the mass of the car
a is acceleration
and
KE = 1/2mV^2
where:
KE is kinetic energy
m is the mass of the car
V is the speed of the car
The energy it takes to accelerate to a certain speed is exactly equal to the kinetic energy at that speed (less any spent to over come rolling resistance and aerodynamic drag).
How fast you accelerate depends on how fast you can deliver that energy. Higher HP means faster acceleration, but it doesn't change the amount of energy required.
The energy to accelerate my Ioniq 5 to various speeds looks like this:
Here's the HP effect on acceleration
This chart shows acceleration times based on getting full HP to the wheels from start all the way to 60 mph. Real world acceleration isn't this good for a couple of reasons. One, is at very low speeds, you're limited to the adhesion of the tires. Power = force x speed. If speed is very low, the force is very high and tires will spin - a "burn out". An ICE engine with a mechanical drive train just can't put out full HP from the start. Full HP usually requires full throttle close to the "red line" of engine speed. At lower RPM, the max HP is lower. An EV can put down the full HP almost instantaneously and hold it all the way through the speed range.
Back to the energy...
The 0.21 KW-hr to 60 mph doesn't look like much compared the 3.5 it takes to move the car a mile at 60 mph, but driving involves lots of braking for stop signs, traffic lights, traffic, etc. Every time you slow down, you have to spend some energy to accelerate again and that energy is later destroyed as heat from braking.
If you can capture that braking energy instead of burning it off and use it to reaccelerate, your economy will improve. EVs and to a lesser extent, hybrid cars can do this. It's called regenerative braking.
Regenerative Braking
It works like this. EVs have a motor that is attached directly to the wheels through a reduction gear set and a differential.
When you put power to the motor, the wheels propel the vehicle.
When you don't apply power, the motor just spins freely, with a very, very, very slight drag from friction and you basically coast.
But when you connect the motor to a load and let the wheels drive the motor, you get braking. You are running the motor as a generator. If the load was just a big resistor, you'd just make heat. But, regenerative braking connects the motor to the battery and recharges the battery instead of just wasting the energy.
The concept is simple, the execution is hard. You have to be able to charge the battery at various rates depending on speed and how hard you need to brake. You need to have the "regular" brakes on the car available if you need to braking that exceeds the system's ability to feed power back into the battery. And, you need to be able to manage all this with an accelerator and a brake pedal and have is feel like driving a "regular" car.
The really good news is the cars manage all this for us sophisticated software and provide it whether we go "new school" and use "one pedal driving" or "old school" and step on the accelerator and brake pedal.
If your trips involve a lot of braking, you will save a lot with regenerative braking. If it's a long stretch on freeway, not very much. More later....
Going up (and down) hills
Hills also require energy to get up. It takes roughly 20 lbs of force to equalize the force of gravity pulling a one ton mass back down a 1% grade (a 1:100 slope, or one foot up for every 100 feet forward). However, any route that takes you back to where you started from, eventually, has the same amount of uphill and downhill elevation change so, you would net out zero extra energy from hills.
However... if you have to apply the brakes to hold speed on the downgrades, you are burning off more energy as heat.
Hilly, local roads with lower speeds and grades above 2% will allow some regenerative braking. Long, interstate trips with grades generally below 2% provide little. There is rarely enough downhill pull to exceed the aero and rolling resistance.
Any route where downhill braking occurs, the EV has an additional advantage over ICE cars. But, we'll ignore that in the simulated trips - later.
Propulsion
This is really simple for an EV and really complicated for an ICE car.
The EV has a battery connected to a power supply, connected to a motor or two, that drive the wheels through a conventional differential, shafts and joints. The performance of this equipment is very high efficiency and varies almost none regardless of speed. There is just a bit of loss in the battery, wiring, motor and drive train. It's in the range of 95%.
Gasoline engine drivetrains are more complicated. The problem is matching the output of the engine to the demand for propulsion. Cars need to go fast and slow, accelerate hard or slowly, climb hills and idle at a stop. The ICE car has an engine that is a heat engine, constrained by the laws of thermodynamics, whose performance and efficiency vary greatly with speed and throttle setting and requires a variable speed transmission to be able to power the car at all speeds and conditions - starting - cruising - uphill - fast - slow. The efficiency of the engine typically varies from 15% up to 30% depending on RPM and throttle position.
Part of the reason the low efficiency is so low is a fundamental part of a gasoline engine - the throttle. You control the power of the engine by adjusting how much air you let into it. When the throttle is mostly closed, the engine gets little air. Open, the engine gets a lot of air. The engine always has to have the exact, right mixture of air and fuel for combustion to occur.
The side effect of having a throttle, is the engine has to "suck" the air past the restriction of the throttle. This wastes a lot of energy. It's called pumping loss. Manufacturers have tried hard to minimize this by installing transmissions with more gears - up to 10 speeds - or even CVTs - with infinite "speeds" so they can keep the throttle as open as possible at relatively low HP cruising speeds. They also have started using smaller displacement engines with turbochargers and direct injection to further reduce "pumping losses".
Even so, the very low power demands of lower speed "city" driving have very low efficiency, while highway driving with it's higher power demand, is somewhat more efficient.
This is an engine map of a 2008 Toyota Camry. The x axis is the engine speed in RPM. The y axis is torque, which is a good proxy for throttle position (higher = more open). The curved lines with the scale on the RH side, are power (in KW - one KW = 0.746 HP) The rings in the middle are engine efficiency (in percent)
The red circle is typical urban driving. Orange, suburban, Green, highway.
You can see urban driving in the 10-15% range. Suburban in the 15-20% range and highway in the 20-27% range.
The Tuscon we're using for comparison has a turbocharged, direct injection engine and does a bit better than this Camry. But, the principles remain the same.
Putting it all together - Some simulated trips.
I created these trips by piecing together moving segments with stops in between each. I assumed steady speed for each segment and zero time to get up to that speed.
For the Ioniq 5:
These assume that all acceleration energy is recouped as regen braking and the whole distance is covered at the speed limit. Terrain is level. Energy is in KW.
This shows dramatically the effect of regenerative braking.
Without regen braking, the economy is right around 3, plus or minus a bit. In urban driving, it's almost double with regen. On highway trips, it has almost no effect - there is very little braking energy to recover relative to the miles and miles of aerodynamic drag to overcome.
Now, for the Tuscon.
This shows exactly what many of us are familiar with. Best mileage on the highway. Worst in local driving. Really fast highway driving starts lowering fuel economy. But, the overall effect is a rather flat curve. The difference between suburban driving and highway driving is minimal.
The two big factors driving this curve are:
Lack of regenerative braking. - lowers urban and suburban driving as so much energy get wasted by braking.
Low gasoline engine energy efficiency at lower speeds despite the best engineering to match engine performance to driving conditions with multi-speed transmissions.
Conclusion
EVs beat ICE cars primarily because they are efficient with the stored energy in the battery. Nearly all the energy stored on board is used to move the vehicle versus only 15-30% for an ICE car. Secondarily, the availability of regenerative braking drastically improves economy in local driving.
The weight and aerodynamic differences have little impact on overall economy.
Bonus Info!
Range and Charging
If you start talking about EVs, the first question that always pops up is about range.
People driving ICE cars are used to filling their tank and driving until near empty and then buying more. There wasn't much to think about. Gas stations are everywhere, have virtually the same price, and all cars fill in a matter of minutes regardless of make or model. Range and price don't depend very much on where you fill your tank or where you are driving.
EVs are different. The cars are all different and have widely different fast charging rates. Energy economy can vary a great deal with the type of trip. The chargers are different by type and even within type. The cost for a KW-hr of juice can vary widely between charger brands and time of day. And, the battery's charge rate can slow a great deal as it approaches "full".
So, charging requires more planning and thought and knowing your range becomes more critical.
Let's zoom in. My Ioniq 5 EV has a 77.4 KW battery and is about 95% efficient getting the juice from the battery to the wheels.
Here's what the economy looks like at steady speed.
Steady 30 mph, 5.5 mi/KW-hr, steady 60 mph 3.5 mi/KW-hr. That's a really wide range.
What range would that 77.4 KW-hr battery give me at various speeds?
400 miles at 25 mph. 200 miles at 75 mph. A really big range.
This shows the problem the engineers have in estimating the range to display on the dashboard. They have no idea how fast you are going to go, they only know how fast you've been going. Using recent history to estimate is the best they can do. So, if you've been doing a lot of local driving at 40 mph, it might show 350 miles range for a full battery. If start going a steady 80 mph, you're actual range is only 190 or so.
My car will often tell me I have 240 miles range with an 80% charge after I've been suburban driving. But, I know, if I jump on the interstate and start driving in the high 70's, I only have about 170 miles range.
Fortunately, having an AC charger at home means not having to worry too much about range. My practice is any day where I'm going more than a few miles from home, I'll charge up to 80%. There's no advantage to not letting it get low before you charge. You're not saving a trip to the gas station! If I'm heading on a long trip, I'll charge it to 100% the night before.
How does one navigate charging on a road trip? The key is planning tools. They are basically navigation systems that will plan your charging stops. They allow you to filter for the charging station types and brands, your tolerance for how low you care to let your charge get, and what features you need at the stop, like food or restroom.
The planning will estimate based on the route and roads you'll be travelling, adjusting for your energy economy. It will tell you what it expects your state of charge to be on arrival and the minimum you should plan to charge to.
Tesla has the best route planning tool. Hyundai and Kia's keep getting better. A Better Route Planner (ABRP) is an app you can run from your phone and actually have it connect to your car so it knows your state of charge in real time.
Some tips and best practices for road trips:
For lowest cost on trips, charge to 100% using your home charger before your leave.
Stay at hotels that have free level two chargers and charge to 100% overnight.
Don't plan on charging over 80% at fast DC chargers on your route. Charging slows down a lot after 80%.
Don't worry too much if you have to use a 150 KW charger instead of a 350+ KW. The difference in charging time is small.
Plan your rest stops around charging. Most times your "bio-break" won't be much shorter than the charging time.
Remember that two fast DC charging sessions from 50 to 80% take the same time as one from 20 - 80% and one charging session from 20 to 80% is shorter than going from 60 to 100%.
If your car came with free charging at some fast DC charging, make sure you filter your route planning tool to take advantage of it. Fast DC chargers generally are expensive comparted to home chargers, costing the same or more than it would to fuel and ICE car.
Engine map. reason for transmissions. migration to many "speeds" CVT. Pumping losses clobber efficicency - engine braking.
References:
2008 Camry
22 HP for 60 mph about 2200 RPM
The thermal efficiency map for the I4 engine from a 2008 era Camry (derived from the I4 engine used in Ricardo baseline simulation for a standard car) (AKI 93 fuel)
Rolling resistance
Where:
- cR = coefficient of rolling resistance
cD = drag coefficient
m = mass of vehicle [kg]
A = frontal surface area [m2]
g = 9.8 m/s
r = density of are, 1.2 kg/m3 @STP
A typical value for the coefficient of rolling resistance is 0.015. The drag coefficient for cars varies, a value of 0.3 is commonly used.
grade resistance 20#/ton
https://www.omnicalculator.com/physics/drag-equation#what-is-the-equation-for-drag-force
https://www.calculatorsoup.com/calculators/physics/kinetic.php
