Great set of experiments, data, and conclusions.
I’m curious about another set of variables, and I’m wondering if you might be persuaded to address them at some point in the future. As a fellow Jeep owner (‘00 5spd Sport), I’m wondering about how far the windows-vs-AC line of thought could be taken.
I’ve noticed that my fuel efficiency improves in the summers (from about 19mpg to about 20.5mpg). No doubt some of this can be attributed to warmer temperatures and seasonal fuel mixtures. But I’m also wondering if having the top and doors off can affect fuel economy too.
I assume that the increased drag from having windows down comes from the fact that air brought into the windows will have no escape, making the entire vehicle a parachute. But in a vehicle like the Wrangler, where the top and even doors can be removed, there is significantly greater opportunity for that air to escape, making (in my uneducated mind) the primary aerodynamic obstacles the windshield/grille and undercarriage.
Additionally, I have a duster (deck cover) installed, which I presume reduces air pressure against the inside of the tailgate.
Do you have any thoughts on all of these variables, and do you have any interest in testing them?
There’s a resident discussion member wiser than I in this thread who may be able to provide the real answer, but let me take a stab.
Regarding the increase in fuel efficiency during the summer, I am going to make two conjectures. The first being the gas change. I cannot remember whether they reformulate during the summer or during the winter – but I want to say winter. In the event that its winter, the added ethanol would decrease the fuel efficiency for the Jeep. Another factor is of course the air temperature. The intake temperature without a snorkel is roughly 20~25 degrees warmer than ambient air. Colder air is denser within an engine and therefore more air is moving through your intake manifold in winter than summer – with a corresponding increase in gas (by the stoichiometric ratio).
I’d love to test top theories if I could convince Quadratec to supply me with a plethora of parts! But I do have a conjecture on the windows/door/roof hypthesis. To start with, I cannot remember where, but I read that having the top completely off the Wrangler really doesn’t impact on the gas mileage much – although it is slightly worse. Why? The hard top with the windows up definitely has the lowest coefficient of drag. You’re also right about having the windows down creating a ‘parachute’ effect. The soft top goes a step worse because it flaps – altering the aerodynamic signature constantly and in direct proportion to any increases in speed. Now taking the top off completely, one would think eliminates the parachute effect. But now consider all of the additional elements that create swirling wind vortices. Passengers. The rollbar (more significant than you might think). The tailgate. Even the back seat! Before those items get dismissed as too small to be significant, think how much gas is saved in a pick-up by lowering or hiding the tailgate. Is the back seat much different aerodynamically? I mention the rollbar because it’s definitely ‘out there’ in the wind stream. It’s also much thicker than your standard roof racks made by Thule. My old Toyota Camry (‘98) used to get 44mpg consistently until I put a roof rack up top – now it only gets about 28 to 30 (this drop was immediate after installing the roof rack). I wager the Jeep’s exposed rollbar will suffer similar aerodynamic consequences.
Speaking of the Camry – that is what bothers me about the compact and efficient car hype today. It IS all a function of how you drive. I drove that car just over 800 miles on a single tank of gas (true, the fuel light was on and I was getting nervous) several times. And that was eight years ago. What have the Japanese figured out so well that Americans keep struggling with? Just the other day I was driving a Ford Focus (rented from Avis – I would never have the P.O.S. associated with me otherwise) and was appalled at the horrendously bad efficiency. For such a crappy car, you would think it would do SOMETHING right. Why else would anybody spend money on something so small, ugly and weak?
Before those items get dismissed as too small to be significant, think how much gas is saved in a pick-up by lowering or hiding the tailgate.
I’d always heard that, but never believed it. Why not? Because when I was a kid (born 1954), I occasionally worked on and around my grandparents farm, as well as their neighbors. When riding in the back of pickup trucks, I noticed that my hair blew forward, not back. This told me that all the guys I see driving around in pickups with cargo web tailgate replacements had quite possibly bought into some bad science. But I wasn’t sure I was right.
Then, sometime (and somewhere) in the last few years (I can’t recall; was it on Myth Busters?), this idea was tested and found that a pickup with the tailgate lowered got less fuel mileage, not more.
Don’t flame me, I didn’t perform the test, but I assume that, had I, I would’ve drawn the same conclusion.
Mind you, I don’t know how much worse we’re talking here, but it was, as I’d suspected, less.
I’d like to take a stab at portions of this…
There are numerous reasons why season affects efficiency. One you already pointed out is the difference in formulation of the fuel during winter and summer months (more light, volatile aromatics in the winter for better vaporization, less in the summer because the heat makes the fuel vaporize nicely), as pointed out once by Rob Rapier. He has something on his blog that really describes this quite well. I think this is where Rob describes the winter formulation, but I’m not sure if that was the best one.
In another part of the thread, you also correctly state that the engine burns a richer fuel mixture as it’s trying to get to operating temperature range. Kind of funny that people jump in cars, drive them until they just reach the temp they were designed to run at, and then shut them off every day. No wonder people don’t reach the EPA listed ratings on their cars. BTW, I have a friend with a Prius that tells me that the engine always runs if you have the heater on because the heater gets its heat from the engine. Normally a Prius engine only kicks on when needed, so your fuel economy is substantially worse in the real world if you run the heater. You also correctly pointed out at one point in this thread that all of your engine and drivetrain lubricants (transmission fluid, differential, bearing grease, etc.) are more sticky and more viscous when they are cold.
Another factor is that the rubber in your tires is more rigid and has higher rolling resistance, which is greatly exacerbated if you don’t keep them properly inflated because the "bump" (deformation from the weight of the vehicle) you are pushing around the circle is bigger and more "steep" if the tire isn’t kept properly inflated.
Cold air is more dense, which is good for your engine, because you can pack more of it into the engine more easily for efficient combustion, but you also have to push through it, which has a tiny effect on aerodynamics. Warmer air is less dense, and thus you can glide through it a tiny bit easier, but I would have to do some back-of-napkin analysis to see if this is anywhere near significant, like it is for aircraft. This is why many aircraft fly at the altitudes they do, and why an F-15 can fly mach 2.5+ at optimal altitude, but just over mach 1 at sea-level—it has to push through denser air. At very high altitude, you don’t have a thick atmosphere to plow through, but your engines are working harder to pull in oxygen to produce power too. Cold air also improves the potential efficiency of a Carnot Heat Engine (a theoretical engine model predicting how efficient you can convert heat to work based on the max and minimum heat). The Carnot cycle is (TH – TC)/TH100%, where T is measured in Kelvins. The TC is the ambient air temp, and the TH is the max temp of your heat source (in this case the peak combustion temp in your cylinder). Theoretically your engine should operate more efficient thermodynamically, but in the real world the difference between summer and winter ambient air temps is small compared to the difference between them and the combustion temp, and the increased in theoretical efficiency is more than offset by the aforementioned fuel reformation for the winter months, the extra engine and drivetrain resistance and friction, etc. This is partly why your snorkels, ram air, and heat shields really help, is because you get the cooler, denser air into the engine (both because of the air density and temp) and it can expand a bit more when it’s heated. Boyles Law is also instructive to understand how the gases interact (the relationship between pressure, temperature and volume of a gas). PV=constant where temperature is held constant, V=cT where pressure is kept constant, and PV=nRT for an ideal gas.
As for compact cars, it’s weird, but US companies seem to equate small cars with "cheap", or people who don’t give a crap about quality, they just want "cheap". Ride in a Prius, or a Volkswagon or other small car and some countries’ design philosophies seem to understand that some people are willing to pay for quality, but don’t want a monster car. US automakers seem to assume that the more affluent you are, the more expensive car you want, the bigger you want it to be. I just don’t get why they can’t figure out that some people perceive value in small cars built with quality.
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