Range / MPG Estimations for a Hybrid
So now that a basic understanding of energy and power has been established, approximate performance estimations of a hybrid may be calculated.
If our example vehicle travels 65 miles at 65 mph, approximately 20 hp per hour or 14.9 kWh are required. The 5 kWh battery pack is rated to deliver 5 kW for 1 hour. If all the energy is used evenly during the hour, then 5 kWh will be delivered by the batteries and the remaining 9.9 kWh must come from the internal combustion engine (ICE) and the gas it uses. The fuel savings (or increase in mpg’s) is approximately (5/14.9 x 100%) 34%. In order to account for energy lost to heat, couplers, drivetrain etc., an electric drive system efficiency of 70% for the home built vehicle is likely appropriate. So a true mpg increase is likely closer to (34 x 70% eff.) 24%. Not too bad for a system that might also add 10 – 50 hp when charging more frequently!
one step at a time
There are a number of complex ways to estimate the hybrid’s range and resulting mpg’s. However for the home builder, an appropriate simplified version is explained below, followed by actual numbers for the Mustang.
With regards to electricity, the amount of energy that is instantaneously delivered is measured in watts (W), or for something the size of automobile, kW. This is a measurement of power, similar to hp.
1 hp is equal to 0.746 kW
A typical mid-sized automobile might take 20 hp to drive down the freeway at 65 mph. That same automobile at 3000 lb will take approximately 150 hp to accelerate from 0-60 in 8 seconds or more aggressively, 300 hp to accelerate from 0-60 in 4.7 seconds. For these examples, the following relationships exist:
20 hp is equal to 14.9 kW
150 hp is equal to 112 kW
300 hp is equal to 224 kW
The amount of power that can be delivered over a duration of time is defined as energy. Aside from racing conditions or driving with a limited ability to refuel, it is rare that a measurement of hp per hour will be presented. However due the limitations of recharging a hybrid battery pack, kW delivered over the course of one hour or kWh is a typical parameter defining how long the pack can sustain a given power requirement. So in our example, assuming no battery loss / heat etc., a 5 kWh battery pack many sustain the above power requires for the following times:
5 kWh / 14.9 kW (20 hp) = .34 hours (20 minutes)
5 kWh / 112 kW (150 hp) =.041 hours (2.5 minutes)
5 kWh / 224 kW (300 hp) =.017 hours (1.3 minutes)
So if the electric car, White Zombie can run multiple 10 second quarter miles on a single charge, can you estimate how much energy that machine is carrying? I’ll give you clue… a lot!
Real Life Example: The 1966 Hybrid Mustang MPG, with Regeneration
The above calculations don't account for battery regeneration. The below example does.
The base vehicle, with the electric motor powered down, generally sees around 17 mpg (this test was with the larger diameter tires, now replaced with smaller diameter tires).
This particular hybrid installation allows for braking regeneration AND regenerative charging directly from the gas engine. Given that the inline 6 burns 1 oz of fuel each minute while idling at a stop light, regenerative charging can make a big difference.
The results are dependent on the electronic / gas throttle start points and ramp up rates. For example:
•Configuring the electric throttle to engage at nearly the same location or just after the gas throttle will achieve 22-24 mpg’s with over a 40 and over an 80 mile range on two separate tests. This setup seems to be appropriate for a non-plugin hybrid or a longer range plug-in.
•Configuring the electric throttle to engage a slightly before the gas throttle and reaching full power quicker seems to achieve an mpg in the upper 20’s to mid-30’s but the hybrid range is reduced to approximately 20 miles. This setup seems to be in line with a plugin hybrid (yet with only a small 2.8 kWh battery pack)
There are a number of expensive modifications that can be made to the Ford inline 6 that might bring the base mpg’s up to the mid to high 20’s. The current goal is to hit the low 20’s with the base inline 6 and continuously adjust the electric programing until optimal settings are reached.
The bonus goal is to successfully implement any of the thermal recovery systems under development.
Hybrid EV'ing to my day job!