Hi,
You may be ready to understand the drag formula:
Quote:
Originally Posted by ShadeTreeMech
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While driving my max, I noticed the scan guage tells me only 50 hp (or less) is required to keep the vehicle moving. I've also learned that pumping losses are one of the biggest fuel economy disadvantages of the throttle body equipped gas engine.
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One of the earlier, vehicle drag formulas had just two terms:
D = 190 + 0.42*(V*V) :: NHW11 drag formula from Toyota sales material
D = C + B*V + A*(V*V) :: the formula the EPA now uses
V = velocity
A = constant 1 (aerodynamic drag)
B = constant 2 (typically transaxle drag)
C = constant 3 (usually rolling drag)
190 = NHW11 rolling drag, typical
0.42 = NHW11 coefficient of drag times area, typical
Regardless of the drag formula used, you can calculate the power needed to sustain a vehicle speed. So let's see how it works in the real world:
- Drag power line (red) - the power needed to keep the car rolling on a flat, no wind condition in HP on the right axis.
- Motive fuel needed, blue - the amount of fuel needed per mile to keep the car rolling at that given speed assuming 31% conversion of gasoline to motive energy on the left axis.
- Motive and vehicle overhead fuel needed, gray - the amount of fuel needed per mile to keep car rolling and powered assuming 31% conversion efficiency. This includes the idle overhead of a car which is independent of speed.
The point is drag, both rolling and aerodynamic are the primary forces the engine(s) have to overcome. But at low speeds, vehicle overhead becomes the limit. What you are proposing, carried to its logical extreme, is a very large number of small but very efficient engines. These engines come on as needed to provide motive power or are off. The two-engine solution, one being half the power of the other, is a similar solution. But the hybrid uses another trick.
Our hybrid-electric cars have a broad power range, efficient engine but there are times when the engine power provided exceeds the vehicle demand while remaining efficient. In these cases, the engine banks this extra power in the battery so later, the engine can be off instead of running in lower power, inefficient bands.
The battery can do one more trick that no fuel engine can accomplish, bank kinetic energy from either braking or while descending a hill. But this could also be done using a hydraulic system, which has some desirable, peak power capabilities over current battery storage systems. My understanding is some formula one cars are looking at a flywheel-electric system but no consumer production versions, yet.
Universally, reduce drag both rolling and aerodynamic, and everyone wins. But in power systems, we're all fighting the 2nd Law. The hybrid has the ability to bank power and extend their efficient power bands while minimizing inertial losses.
Bob Wilson