starting engine temp vs ambient temp re: winter MPG

Former Member 68813 · Dec 14, 2012

Here is my observation re: winter MPG.

My trip MPG doesn't suffer noticeably in my very predictable 10 mile city commute until ambient temps slip under 60F. My normal MPG is 60MPG (indicated), goes near 55MPG 40-60F and 50MPG under 40F.

However, if I keep the car in attached, insulated garage overnight and the starting temp is near 60, there appears to be no MPG penalty even if the ambient temp is near 40F. Keep in mind, that I use no cabin heating at these mild temps, just a coat.

Based on that, I suspect engine block heater and not grill blocking is the way to go to improve MPG in winter.

Sergio-PL · Dec 15, 2012

MPG suffer depends on battery temperature and engine heat-up time.
Recently I filled-up car a day after temperatures dropped below freezing. Somewhere between -12 and -4 Celsius (10 - 24F).
After about 300 km (187 mi) I refilled the car. Result: 6.9 l/100km (34 MPG) while normal MPG is about 47 - 50.
I'm not using any block heater and the car stays outside all the time. In mild temperatures, just over freezing not using cabin heater improves MPG just like in your observation.
But if battery goes below freezing enabling heating gives positive result. While battery is cold even slow crawling in traffic jam requires ICE working, no battery power at all. After it preheats then works fine.
My commute is very short - only 10km (6.2 mi) one way. Most in traffic jam.
EU Prius is different from US Prius in one thing. It will shut down engine in S2 state.

Former Member 68813 · Dec 15, 2012

Good point. I forgot the battery temp issue. Looks like heated garage is the best solution.
I'm glad I at least insulated the garage last spring.

Codyroo · Dec 16, 2012

Grill blocking will limit the amount of cold air that hits your radiator and engine, allowing the engine to warm up more quickly. As a consequence of this, you will hit the most efficient S4 state (159F) sooner. Also, once above 159F, you can turn on the heater and warm up the cabin (and the battery) which should help further still.

F8L · Dec 16, 2012

You will always have a mpg penalty in lower temperatures. The colder the engine/battery the worse the hit will be but beyond that you still have to cope with higher rolling resistance of the tires and road surface. If driving at high speeds then you also have to overcome increased air density thus increased drag.

Former Member 68813 · Dec 17, 2012

F8L said: ↑

You will always have a mpg penalty in lower temperatures. The colder the engine/battery the worse the hit will be but beyond that you still have to cope with higher rolling resistance of the tires and road surface. If driving at high speeds then you also have to overcome increased air density thus increased drag.
Click to expand...

I still think the engine/battery temp impact is huge compared to the tire/air impact (if one adjusts air pressure in cold temps).

F8L · Dec 17, 2012

friendly_jacek said: ↑

I still think the engine/battery temp impact is huge compared to the tire/air impact (if one adjusts air pressure in cold temps).
Click to expand...

I was focusing on rolling resistance due to reduced pliability of tire rubber and other components as well as road surfaces. Tests have shown that rolling resistance can almost double when temps drop from 100F to 32F (approx.). This is independent of air pressure and assume the same pressure in both temperature extemes.

More on rolling resistance.

Tire Review - Rolling Resistance: What Really Makes a Tire Fuel-Efficient?

The more a tire at a given pressure is loaded, the more it deforms; hence, hysteresis increases with wheel load. Indeed, the relationship between rolling resistance and sidewall deflection due to load is approximately linear, so increasing the load on a tire results in a near-proportional increase in total rolling resistance. As described later, this linear relationship allows rolling resistance to be expressed as a coefficient with respect to load under normal operating conditions.
Inflation pressure affects tire deformation. Tires with reduced infla- tion exhibit more sidewall bending and tread shearing. The relationship between rolling resistance and pressure is not linear, but it is consistent enough for rules of thumb to be applied. Schuring (1980) observes that for conventional passenger tires, an increase in inflation pressure from 24 to 29 pounds per square inch (psi) will reduce rolling resistance by 10 percent. For a tire inflated to pressures between 24 and 36 psi, each drop of 1 psi leads to a 1.4 percent increase in its rolling resistance. The response is even greater for pressure changes below 24 psi. Maintenance of tire pressure is therefore important in preventing excessive deformation and hysteresis, as well as in achieving intended wear, traction, handling, and structural performance.
The temperature of a tire is affected by ambient conditions, tire design and materials, running time, and speed. Higher ambient temperatures are associated with reduced rolling resistance because the amount of energy dissipated when the rubber is subjected to repeated deformation declines moderately as temperature rises, which is a commonly observed behav- ior of viscoelastic materials. Accordingly, the length of time a tire has been running since the last cool-off affects rolling resistance, which declines until the passenger tire has been rolling for about 30 minutes. At that point an equilibrium temperature is reached and rolling resis- tance stabilizes.

Road Surface and Hysteresis

Researchers have known for some time that rough road surfaces con- tribute to rolling loss by exacerbating tire deformation. This effect can in- crease energy losses by 5 to 20 percent (Velinsky and White 1979; DeRaad 1978). Road roughness has two components: macrotexture and microtex- ture. The first relates mainly to the surface condition on a scale of inches to feet and reflects the presence of cracks, ruts, bumps, and other surface irregularities. Macrotexture can include intentional changes in surface tex- ture, such as surface grooving to improve water runoff. The second com- ponent, microtexture, relates to smaller-scale asperities in the road surface that are millimeters or even fractions of a millimeter in size and reflect the coarseness of the surface texture. Tires operated on a rough macrotex- ture or rough microtexture will deform more and suffer greater energy loss. They will also experience faster tread wear.
The roadway can also contribute to rolling resistance by deflecting or deforming under the weight of the wheel load. How much energy is lost will depend on the rigidity of the roadbed and overlay. Dirt and gravel roads de- form the most and give rise to twice as much rolling resistance as harder paved surfaces (DeRaad 1978). However, most driving occurs on paved surfaces, which can vary in rigidity depending on the overlay, base, and sub- grade. The most rigid, or nondeformable, pavements tend to be those with a concrete surface layer and reinforced base, followed by an asphalt surface on a concrete base, and an asphalt surface on a compacted gravel or soil base. The rigidity of asphalt overlay depends on the amount and type of asphalt used in relation to aggregate and on environmental conditions such as temperature. A rubber-modified asphalt overlay (often derived from the ground rubber of scrap tires) will deform more under load and thus should create more rolling resistance than harder asphalt pavements.
Click to expand...

http://onlinepubs.trb.org/onlinepubs/sr/sr286.pdf

starting engine temp vs ambient temp re: winter MPG

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starting engine temp vs ambient temp re: winter MPG

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