Incredible Yaris Hybrid city fuel economy

Prius veterans remember a fantastic fuel economy marathon some years ago using a G2 Prius. The car was filled to the lips with fuel and driven until empty using local roads on a ~ 30 mile or so circuit. Every couple of hours one of four (or was it five?) drivers would take the wheel. IIRC the car was done at around 1400 miles; the highest MPG leg was about 125 mpg by Wayne Gerdes. That record has since been eclipsed in the US, while similar records have been performed in Japan.

So 140 mpg is possible, and it does not mandate downhill driving. It does imply either good wind or slow hypermiling. Certainly a round-trip value is more impressive.

Regarding the possible descent: If we start with 1200 kg mass, every 100 meter descent is mgh = about 1,200,000 joules or 1/3rd kwh of potential energy. If the car ran at best thermal efficiency of 38%, a 100 meter drop saves 0.876 kwh of fuel, or about 0.026 of a gallon. These drivers used about 0.6 gallons, so a 100 meter descent would improve their fuel fuel economy ~ 4.5%.

 

I get with normal driving from 3.6-4.2 liters per 100 km with Yaris HSD, this is not bad at all. Very good car indeed, typical fast moving to downtown gives us the following statistics: see image, it all depends on routes, the described route has lot of traffic lights and I sometimes get 35 regenrative braking signs to my statistics histogram.

 

The mileage log was available so I added it to my MPG vs mph chart:

The black dots in the 24-33 mph range are from that marathon drive. If I remember correctly, Ken@Japan provided one the earliest, higher MPG instrumentation packages that provided the detailed metrics in the chart. More of the story can be found here: Prius Marathon: 110 MPG in a non-plug-in hybrid

I've been more interested in reproducible results that anyone can achieve primarily as a vehicle diagnostic. So my questions have been:

• What makes a reproducible route?
  • As flat as possible to avoid non-linear engine power modes.
  • Same start/stop points or if not a loop, at each end with a 'run up' to enter at target speed.
  • Paved to avoid non-linear gravel and rough surface effects.
  • Distance of 10 miles averages the Prius flipping between gas and battery power.
• What weather?
  • Dry because rain puddles vary resistance too much.
  • Wind as low as possible, 10 mph or less, as aerodynamic drag varies by the square of the velocity. A vector sum problem, slightly higher winds can be used if a crosswind. Benchmarks between midnight and dawn work best but there is a slightly higher electrical load due to lights.
  • Temperature above 45 F as earlier Prius have control laws requiring a minimum 60 C coolant and will burn extra fuel to preserve coolant temperature. Also, the increased air density increases aerodynamic drag.
  • Standard day temperature is 59 F at sea level. There are correction tables for density altitude that can adjust aerodynamic forces for other temperatures.
  • Bob optimum, ~65-85 F depending upon sun and humidity as the driver won't dehydrate and can actually enjoy the benchmark.
• Car preparation?
  • Calibrate the distance and speed metric, I prefer GPS but a highway with mile markers can also be used or a topographical map.
  • Four wheel alignment of camber and toe. Run 1,000-2,000 miles to let the tire wear adjust to the new alignment.
  • Tires at maximum operational pressure, typically maximum sidewall.
  • Engine and transmission oil change and run 1,000-2,000 miles to let normal shear-down work.
  • Switch to a high-energy gasoline, if possible, but this requires metrics. Lower octane gas tends to have more energy than higher octane BUT gasoline really has to be measured and this is not a trivial task . . . I run my tank dry to get accurate metrics.
  • Instrumentation, I prefer mass flow but injector timing and engine rpm works too. Use automated systems to record the values.
• Driver preparation?
  • Nothing else to do.
  • Willing to throw out data if a procedure error is found.
  • There is no deadline that trumps accuracy.

I would also like to share these wise words:

Source: John's Stuff - Toyota Prius Personal Log 215

My only interest in accurate mileage numbers over shorter distances is as a diagnostic tool. We often find newbies reporting poor Prius performance and they are not in any mood to drive a tank of gas over several days to prove the car is not broken. However, a shorter, disciplined set of test drives can quickly determine if it is route, driver, or car.

Bob Wilson

ps. Sometimes I like to have fun:

 

One hypermiling technique that pushes a car over 100 mpg is a hypermiling trick called NiCE -- which basically turns a car (which can do it) into a soapbox derby racer. There are several things that have to come into play for this to work. Not all cars can do Nice; a car must be able to electronically start when the transmission is in neutral, the steering should work in neutral, and the brakes should function while the gasoline engine is off but the transmission is still in neutral (Honda Insights and Civics can do this). To do Nice optimally a car needs a manual transmission (many elite hypermilers preferring a 5 speed Manual). The roadway must be relatively empty when you are driving. Finally, putting your vehicle in neutral while in motion must be legal in the state where Nice is being used. (Driver induced/manually doing) NiCe can be legal or illegal depending on your jurisdiction.

In NiCE a hypermiler gets a car rolling to a top target speed (e.g. 40 mph) -optimally at the top of a hill - and then puts the transmission into Neutral mode and then turns off the gas engine to save gas while the car is still in motion. The car then acts as soapboax derby racer -- running on its momentum and gravity-- until it reaches its bottom target speed ( e.g. 25 mph) . When the vehicle reaches its bottom target speed, the hypermiler restarts the engine while the transmission is still neutral and the car is still moving. Once the engines have been restarted the transmission is switched back to Drive mode and the hypermiler starts the cycle over again. Nice is an extreme form of Pulse and Glide (P&G) which gets its added efficiency by eliminating the internal drag of the motor on the drive train transmission and eliminated the fuel cost of idling/running the motor during a glide phase of the NiCE cycle.

The Prius does Forced Auto Stop (FAS) , i.e. it turns off the gasoline engine automatically under certain situations - which is one of the reasons it gets higher fuel efficiency that most cars. NICE is illegal in my jurisdiction - so I don't do it. The Prius c is much lighter than my 2010 Prius and potentially a hypermiler should be able to get better MPGs with a 2012 Prius c at low speeds (<40 mph).

My best commute sofar from a cold start was 85 mpg (2.8 liters/100 km) on my 2010 Prius. Both my ScangaugeII and my Prius' MFD both registered the same results which was from a summertime test run which involved a 15.8 mile one way commute with an ambient driving temperature of about 65 Fahrenheit degrees that included an overall elevation drop of 200 feet (500 ft above sealevel to 300 feet above sea level) with a sustained speed of about 25 mph using 10% ethanol 87 octane gasoline. The tires were also over inflated to 44psi front and 42psi rear. That summer this particular route in rush hour traffic would normally gets between 65 to 75 mph with the average being 73 mph. To boost the MPG to 85 for that particular test run I drove at 4am-5am when nobody was on the roads and all the traffic lights were essentially turned off (blinking Yellow) - so basically while the Prius was mainly going between 25mph to 35 mph for most of the time - the Prius rarely had to stop.

My guess is that a Prius c - given those same conditions - should be able to get +95mpg.