This is all very good data for large HP engines that don't need a long time to reach op temp's in cold weather. Unfortunately for us 1.3 L hybrids, colder weather spells trouble for FE because it takes longer to reach op temps (richer fuel mix), and when in autostop, the block loses some heat, typical in non prius engines. All of this takes more fuel to keep everything warm and cozy. With small block engines, it makes sense to pre-heat intake air or use block heaters to preheat the coolant in cold weather, which is one way to increase FE. We aren't looking for increased HP because we have a high torque electric motor to augment acceleration from a dead stop so all bets are off. As far as using different octane fuels, there have been many threads about this subject and my personal experiences have indicated that 87 works fine in all climes in a small block. Vapor pressures of lower and higher octanes are different and will probably make a slight diff when trying to start a non-fuel injected (carburated) big block in severe cold or hot temps.

 

I have a Highlander Hybrid and I believe that a larger engine relatively speaking may not be impacted any less than a smaller engine during warm up. In fact, since the engine is larger, there is more mass to heat up. My MPG suffers during cold start-up and the actual dollar cost of this start-up inefficiently is higher for my larger engine.



I agree that a block heater to pre-warm the coolant may be a good idea to reduce this cold start inefficiency. For the Highlander, the block heater consumes 600 watts. 3 hours running in moderate temps (unheated garage) and about 5 hours in winter before the morning commute would cost $.75 and $1 respectively per week used 5 days at $0.08/KWH



Gasoline engines only convert about 30% of the fuel consumed into mechanical energy to move the vehicle. 70 % is converted into heat that is dissipated by the radiator, heater, and drive train by convection. It’s a little comforting knowing that in cold weather, a bit more of that waste heat can be reclaimed for the useful purpose of keeping the passenger compartment warm.



The increase in available HP due to cold weather can provide faster acceleration (and lower MPG from this aggressive driving behavior) as you suggest. However for those focused on higher MPG, I believe that colder intake air improves efficiency in an engine that has reached operating temperature.

The pistons will produce more power for every cycle when fed with colder, dense air. For a given load on the engine, this allows the engine to run at a lower RPM and still put out the required power to match the load. When the engine runs at a lower RPM, pumping and friction losses are reduced.


Note that MPG will tend to be less in the winter due to lower energy winter blend fuels that all gas stations switch to when cold weather arrives:



Average Energy Content in Gasoline (source: US EPA)

Summer-- 114,500 BTU per gallon
Winter-- 112,500 BTU per gallon
Difference-- 1.7%

To further affect your fuel mileage, all gasoline varies widely in energy content from batch to batch and station to station. Your next fill-up may affect your MPG:



Energy Content Variation of Gasoline
(BTU/gallon) (source: US EPA)

…………….Minimum……Maximum…….Difference

Summer………113,000……..117,000……..3.4%
Winter…………108,500……..114,000……..4.8%

Concerning the use of prtemium fuel, the fuel grade to use for best efficiency is dependent on the how the engine is designed. If the engine is not designed to advance the ignition timing with higher octane fuel, no MPG improvement will be achieved unless the combustion chamber has built up detonation causing deposits that force the engine to retard the timing.

In my Toyota Highlander Hybrid which was designed to benefit from premium fuel, the engine will advance the ignition timing to more optimal efficiency settings with premium fuel resulting in both more available power and greater MPG.

Most hybrids have CVT transmissions which are programmed to optimally keep the engine under an efficient heavy load or shut it off. The motors are there to facilitate the positioning the engine in it’s most efficient operating range for the greatest feasible time.

Since the engine is typcially running under moderately heavy load for max efficiency, the octane reqquirements of the engine is also maximized. so using an octane below the optimal for the engine will have a greater negative impact on the hybrid engine fuel efficiency than a conventional drive train engine that runs lightly loaded for a greater percentage of time.

 

Hi Don,

We have a record of hybrid electric performance spanning years including vehicles in temperate zones. It is fairly easy to grab any hybrid electric vehicle's record and plot the MPG as a function of temperature. Do you have simular performance data for non-hybrid Highlanders over at least a year of service?

We know hybrid electrics have some unique, cold weather performance challenges including battery and electric heater operation. Furthermore, because of the lower engine power requirement (aka., our fuel efficient engines,) temperature effects on tires, wheel bearings, transmissions and cold weather air drag have significantly greater impacts.

Although you are correct that colder air gives more power, this is the principle of a super-charger intercooler, it is inaccurate, grossly inaccurate, to claim this means higher fuel efficiency in a gas engine vehicle. The ratio of fuel to air remains the same regardless of the air temperature or density. Colder air give more ICE power but also more fuel burn. But I would be interested in actual data showing a different results.

Do you have MPG efficiency data for non-hybrid Highlanders like this:



This example came from:
https://www.greenhybrid.com/compare/...e/car/755.html

I've separated the shorter distance records from the longer distance near the middle of the records. Clearly, both short and long distances are impacted by cold weather. Where is the non-hybrid Highlander data?

Bob Wilson

 

Hi Bob,



My previous post doesn’t conflict with your fuel mileage scatter plot data. Cold start warm-up significantly reduces fuel efficiency and cold weather extends the warm-up time to further reduce efficiency. A block heater would help here. Winter fuel has less energy and therefore this factors in for less MPG. Running the heater to keep the car warm can cause the ICE to operate more often reducing fuel mileage, especially if you are sitting idle in traffic with the engine off for longer periods.



I believe colder intake air has a positive effect on engine efficiency when the engine is operating close to peak torque. Note that this effect may be small and outweighed significantly by the other variables mentioned above. At part throttle where pumping losses become a factor, colder intake air does not improve efficiency. Engines typically have EGR valves to abate some of the pumping losses at part throttle by re-circulating combusted exhaust gasses into the cylinders.

 

First, I don't know about states where you guys live, but I think that Massachusetts has switched over fully from MBTE (which used to be only in the winter) to Ethanol additive all year round. Are there other ways which the fuel formulation changes to lose power in the winter? Or am I confused on some point here

Second, I have a manual 2.0 L (~115 HP) engine. The last thing I need is more horsepower! But you can check out the graph for my car here I clearly get lower efficiency in the winter, by about 5 MPH on average. I bet you are all right to say it has to do with the hit from warming up the engine. I don't know what exactly the car's computer is doing on start up in cold weather, but I can definitely smell richer exaust. Also, the engine has an air pump that only kicks in in cold weather (and if you try to disconnect it, the engine throws a code!).

 

Good thought. As a person who is naturally cold (a help during the summer,) I'm dreading the <30F days. One thing that is reassuring is that the EV is helping out the ICE. If you look at the overall picture, any use of an alternative mechanism is helpful.