Latest fuel consumption improvements: Is it my pulsing or my gliding?


On Sunday I had about the worst type of journey I could have for fuel consumption (FC) a short trip with a very steep climb, fast speeds, a dirt road and cold temperatures necessitating both A/C and rear and mirror defogging.  I returned a 5.0l/100km (5.3l corrected) for that trip.  Today’s work round trip has returned to the very good numbers of late last week.

Date Trip Distance Cons +6.5% Ave km/h
12/06/2012 Home to AIS 12.2 4.0 4.3 42
12/06/2012 Round 33.3 4.2 4.5 38

How am I doing this? Any reasonable person would think that the most efficient way to drive is to maintain a constant speed.  It is certainly more efficient than treating the accelerator and brake as an on/off switch.  Hard acceleration and braking can increase FC by 30% or more without much thought. But there’s a benefit to being smooth but inconsistent.

The key term is Brake Specific Fuel Consumption (BSFC)

Engines are funny things. It seems obvious that revving the ring out will use more fuel, but an engine under load generally uses fuel more efficiently.  There is a balance.  Here’s a very good explanation with lots of pretty pictures, though the author is referring to the original NVW11 Prius when he shows the BSFC curve.  It is a fairly complicated concept and does seem counter-intuitive, but then so is Quantum Mechanics.
Understand one thing; the Prius is pre-destined to run its engine in the area of maximum BSFC and therefore get the most out of each drop of fuel.  The clever computer keeps the engine revs within the 220g/kWh zone as much as possible. A typical reading from the gauges on my Garmin ecoroute HD might be engine load 90% at 1900 RPM.   Driving in this zone will produce gentle acceleration as the revs are low, but you can always put the foot down and lose a little bit of BSFC to gain torque and power if conditions require it.

As a rough guide, keeping the Hybrid System Indicator between 75-100% (that is, from halfway along the “ICE” section to the edge of PWR) will keep the engine speed in an ideal range to take advantage of the lowest BSFC.  This zone will maximise the recharging of the HV battery, which is important for the next point.

BSFC graph for 2ZR-FXE 1.8L Prius compared to the 1NZ-FXE 1.5L. The larger “sweet spot” of the 1.8 is obvious.

I think that I have finally cracked the secret of “Pulse and Glide

P&G is a hypermiling technique that is particularly well suited to the Prius Hybrid but can be done in most cars.  In short, you Pulse to gain speed or climb a hill and then Glide by getting the car to coast until the speed drops.  If the pulse is reasonably gentle, the engine will be in its sweet spot and the HV battery will get a charge as well.  What’s makes the glide easy with the Prius is that you don’t need to shift to neutral or switch off the engine; the car can do that for you.  You just have to learn how to press the accelerator ever so slightly to avoid its kinetic energy being used to charge the HV battery.  Here’s a video that explains everything:

So it looks like I have somehow cracked the P&G code such that despite cold starts, high speeds and stop-start traffic, I am getting close to the fantasy FC figures that ADR81/02 under fairly poor conditions.  I think that I’ll still buy that engine heater (that’s another post for another time.)

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