2011 Chevrolet Cruze: Hot Weather MPG Part 2
June 20, 2011
The numbers are in. Our 2011 Chevrolet Cruze has returned from Arid-zona and is back in the cooler coastal environs of SoCal.
Before we get to that, I thought I'd share a couple of highlights from a conversation I had with Mike Katerberg, the Assistant Chief Engineer of GM's Ecotec 1.4-liter turbo engine.
First off, Mike thinks you guys rock. He didn't use those exact words, but he said more than once that Inside Line has some knowledgable commenters on this blog.
He confirmed my suspicions about the intercooler for the low-speed launch behavior, but also pointed out that extreme hot weather favors the use of 91-octane premium unleaded. My use of 87-octane in these conditions led to my loss of drive-away power as well the wonky fuel economy over the trip.
In mild weather, 87-octane does just fine, but with high heat comes detonation, and the Cruze's ECU counters by retarding the timing, a move that does engine torque output and efficiency no favors.
At drive-away, where V = not much, my heat-soaked air-to-air intercooler can do little intercooling, allowing hot compressed air into the cylinders instead. The resulting low density mixture prompts the ECU to retard the timing to ward off detonation. The intercooler comes back to life with speed and airflow, but the overall high ambient temperature conditions still bring with them a higher chance of detonation. So there's still less spark advance at cruising speed in hot weather, it's just harder to tell. And you've got no way to feel it happening if you're on cruise control, as I was.
JKav, back from LeMans but still jet-lagged, is drawing me an MBT graph (maximum brake torque) graph showing how turbos benefit from the spark advance that comes with higher octane far more than naturally aspirated engines do. "Turbos like octane," he said. He's explaining the tech behind that "no duh" knowledge.
But this octane sensitivity is even more true of small engines with a small turbo like the Ecotec 1.4T engine. Such mills reside further down the inverted parabolic MBT curve from the theoretical apex point of optimum ignition timing. Anything that allows more timing, such as 91-octane fuel, will make a noticeable difference by allowing torque to march up the steep part of the curve. Jay scribbles a few tick marks with his pen saying, "A little goes a long way."
On the freeway, the cutback in ignition timing reduces engine torque and efficiency for a given amount of throttle, which means that even though I'm not aware I'm doing it I am in fact loading the engine to a higher degree by booting the throttle a little bit more to keep my cruising speed where I want it. Unlike a large-diplacement blown engine, the turbo on a small forced-air engine such as this is actually doing something other than pinwheeling while cruising on the highway, meaning it's more sensitive to the above-described ignition timing and efficiency issues.
Enough of the Dr. Science routine already. Here are the overall trip mpg numbers for the Cruze 1.4T to and from Phoenix and the Buick Regal 2.0T to Las Vegas and back.
2011 Chevrolet Cruze LTZ 1.4T automatic
- Phoenix and back, 672 miles
- 87 octane unleaded
- 36 mpg EPA highway rating
- 28.1 mpg trip average
- 111 degree peak
2011 Buick Regal CXL 2.0T automatic
- Las Vegas and back, 578 miles
- 91 octane unleaded
- 28 mpg EPA highway rating
- 30.8 mpg trip average
- 107 degree peak
The weather was more or less equally hot on both routes and both of us used cruise control. Erin apparently had a couple advantages: premium gas and a larger displacement turbo mill.
As for the Cruze, it's certified to run on 87-octane fuel because that makes sense for most buyers most of the year. But that doesn't mean that 91-octane fuel is always the more expensive choice in hot weather. The use of more expensive fuel on this trip would have paid for itself on a cost-per-mile basis if it had pushed average mpg up to just 30 mpg. Anything higher than that and I'd have saved money by buying "expensive" gas for this trip to Phoenix.
This impromptu comparison wasn't as scientific as it could have been -- Erin I basically compared notes and realized we'd driven much the same way in much the same conditions. She happened to use 91-octane because she felt like it.
Look for a repeat test with more of the variables controlled, but it seems that desert dwellers that drive certain small-displacement turbo cars may actually save money by paying for premium gas in summer.
Dan Edmunds, Director of Vehicle Testing