Weight savings - seems far more beneficial in small cars
The Mazda 2 was going from strength to strength last week as its fuel tank emptied, finally delivering 4.2l/100km (US 56mpg, UK 68mpg) on a 60/40 freeway/city run to the airport including quite a bit of dawdling through the airport's giant car park.
Now the tank is full and it is noticeably using more fuel. Putting aside the possibility of fuel batch variation, it made me think the difference weight makes in small 1.5l cars vs my big 3.0 litre turbo 6 cylinder Bmw.
The Mazda just doesn't have the required torque to counter weight variations, and those variations can be quite big in % terms when a car only weighs 1,000kg so a small change has an almost exponential impact? What are your thoughts?
Couldn't agree more, if you've ever filled a 20l Jerry can and felt the weight, a full tank is more than 2 of these. My previous car felt quicker with almost no fuel in. In Motorsport they have small tanks and fuel up often, not just for saftey reasons, but weight saving gains too. My MPG'S start to increase in the last gallon or so as the car gets lighter. Sadly fuelly needs a full tank each time to calculate economy. As you say, a smaller lighter car will be carrying a larger percentage of weight, and be labouring more carrying that weight.
I'm not really mechanically or physics minded, but I do think about such things from time to time, and I agree that it can't be entirely attributed to proportional differences with regards to added weight versus total weight, and I agree that available torque (running torque even at low output levels of larger engines) diminishes the effect of added weight to a large degree; because that torque is there running even while not under this extra load and adding weight only adds slight load, whereas in smaller vehicles, with less torque requires more output to move the weight. This is one of the reasons why diesel engines increase their advantages under heavier load due to the fact that they can bear the load without generating alot more power output as compared to a less torquey engine. TFL Truck recently did a top three towing pickup mpg, and number three out of all pickups tested, was the very powerful and most torquey, 6.2L V8. Most likely, this same truck would come in near last when not under load, but under load, it was near the top in mpg; coming in just under two diesels, and above all other gas engine-powered trucks that are smaller displacement, and supposedly all more FE.
Also, I would think that the more economical a vehicle is, the more any negative impacts whatsoever will diminish that great mileage. Whereas, if a vehicle is already getting poor or fair FE, then those small changes have lesser impact.
But let me suggest an exception and maybe someone more physics minded can refute or support my belief based on science or math... Right now, in North America, the number one selling vehicle; F150 full-size truck is now all-aluminum body panels, and comparing the 2014 models to the 2015 models, Ford has cut 330-750 pounds from the base curb weight, depending on the configuration, and added to that, Ford also introduced a smaller, but toqueyer, twin turbo, direct injection, spark-ignition engine (2.7L V6) that sort of takes the spot of a previous edition power option, what we would have called, a small V8 that were, of course, naturally-aspired. This small Ecoboost, in most configurations, the configurations that most Americans and Canadians buy, is still a pretty heavy vehicle; maybe up to 4800 pounds, and many of these owners are claiming disappointing real-world mpg results, well below the 18/25, 21 combined mpg rating for 4WD standard duty, which means all this engineering has resulted in only average real-world mpg for a full-size truck in these most popular, heavier, more expensive versions of this truck.
However, I've got the lightest version of this truck; 2WD, standard cab, short bed, 3.31 rear axle that comes in at under 4200 pounds, and in this not-very-popular version of this F150 with this new, smaller, but torqueyer engine, I can achieve very close to the EPA rating for 2WD of 19/26 & 22 combined and even though this engine puts out 350 foot pounds torque from 1900-4500 RPM, which should mean that added weight should have lesser effect, it appears that adding weight matters alot in this one example. My thought is that this technology of adding boost, torque and power via turbo charging for spark ignition can be very sensitive to any and all factors that affect mpg, including weight and that's because of the relatively rich air-to-fuel ratio requirement of spark ignition; meaning when those two little turbos spool up just enough to give that toque, even in the absence of high output, it still has high efficiency losses.
Impacts to fuel economy, whether good or bad, within control of the driver or not, result in a percentage change. This means the changes are more apparent when a large value is used to express the fuel consumption.
Along similar lines, any changes in weight need to be viewed as a percentage change also. The in the case of trucks, they are designed to carry substantially more weight than a car. In other words, cars are likely closer to their max weight to begin with. So less weight is needed to push the engine to work harder than in a truck.
The secret to get great fuel economy with a turbo gasoline engine is to keep it out of boost. On top of the more boost equaling more fuel, the ECU might dump a little extra gas into the cylinder to help control temperatures.
Low 65HP cars from the 80's usually had gas tanks not over 10 gallons, & often less, along with 12 inch wheels(lighter weight & less circular momentum), no airbags(cinch up that seatbelt), & thin sheet metal(ya sat on my car & dented it!). But the 57mpg was sweet, when the behemoths were getting below 20mpg. But I really got in trouble with my girl friend with my low horsepower! We were traveling up a steep grade at 4000 feet (losing lots of HP from the elevation), that I often drove by myself. Said I (too late to take it back!), "I can feel the difference in the throttle with two people". Hum.....THAT'S WHY WE NEVER GOT MARRIED!!