Heartland Musings

More Fuelish Thoughts

I often indulge an informal study of how different variables affect fuel economy. I’ve gone so far as to monitor in real time key engine and vehicle parameters from the car’s computers to see how they relate to overall fuel economy .  The evidence—albeit anecdotal observations of empirical data—is so consistent and repeatable, that I find it absolutely conclusive. Excellent fuel economy really comes down to just a few key ideas that matter more than all the others combined.

1)      Stay in the highest gears of your transmission to the greatest practical degree

2)      Coast as much as possible before applying brakes.

The implications of the first point cannot be overstated. There is no acceleration rate or vehicle speed that will give you good fuel economy in first gear. Full throttle in first gear, you might be momentarily getting only 1-2mpg. Steady speed you might only get 4-5mpg. Conversely, the highest gear of your transmission might give you 20-25mpg even at full throttle, and 35-50mpg at a steady state on modest speeds around 60mph.

In other words, full throttle in the highest gear of your transmission will still deliver many times better fuel economy than mere idling in the lowest gear of your transmission. The higher engine loading from the taller gears drastically increases efficiency.

This is why it is it is possible sometimes for a larger engine to actually provide better fuel economy under some conditions. Consider the case of two pickup trucks, one with a powerful V6 and another with a yet-more powerful V8. The window sticker indicates that the V6 model should get about 2mpg better in EPA testing. The EPA cycle, however, does not include towing or steep hills.

If greater engine torque of the V8 at speed prevents or delays downshifting to a lower, less efficient gear, then it’s quite possible for the V8 to get better fuel economy. Experienced drivers who tow large trailers are well aware of this phenomenon, having seen small engines deliver terrible MPG because they must drop down several gears to have enough torque to climb the hill. The loss of mpg from the kickdown is greater than the small gain in MPG from selecting the smaller engine for running on level ground.

This is a major reason why semi trucks that need about 450 horsepower use a large engine (15L) spinning at low RPM and delivering massive torque (1500+lb-ft) when they could just use a late model Corvette engine and rev it up. The Corvette engine would not only consume much more fuel, but it would wear out very quickly compared to the million-mile longevity of the large diesel.

I’ve also found interesting the degree to which the importance of the relative factors changes with the conditions. For example, tire pressure is key factor, but goes from being entirely unimportant at rest, to being super important at around 30-40mph, to being only moderately important at 70mph.

Having your air conditioning operating can go from being dominant at slow speed to having only a 1-2mpg effect at highway speeds. Recent experiments in my own car, with a smallish 2.5L engine, have shown almost 3 mpg difference in stop-and-go city driving between using or forgoing air conditioning. At idle, the air conditioning almost doubles my car’s fuel consumption based on real-time monitoring. My minivan sees far more drastic results because the air conditioning must work so hard to cool a large interior, and this vehicle has both front and rear evaporators. Driving in town went from 17.5mpg to almost 22mpg just by turning off the air conditioning. (OK, it actually went out, but the effect is the same, lol!)

It all matters, of course, but focusing on the most important things is the best way to reduce your fuel consumption.

 

The Minimalist Auto Maintenance Requirement

There are really only three things that must be looked after on most cars to assure reliability, as I definite it by 'probability of leaving you on the side of the road.' Here, in order of importance, are the 'majors to major in" as the saying goes.

Tires are the MOST important maintenance item, by far. Every steering, braking, and acceleration action goes through your tires. Rotating tires is nice, but the tire pressure is the most important thing. It must be maintained. Overinflating your tires may cause uneven wear and a harsher ride quality, but underinflation is the more serious risk by far. Always err on the side of higher tire pressure if you have any doubt about the proper value. Far better to lose a litte tire life than to lose your own life from a loss of vehicle control due to catastrophic tire failure.

The cooling system is the next most important system on your vehicle, and next to tires, is the most likely to leave you stranded by the side of the road. Your engine's coolant level and composition must be maintained to work properly. Regularly inspect the hoses and repair any coolant leaks immediately.

The main reason coolant is so likely to leave you stranded is that many people never service the system or inspect it. Everyone knows you should change your oil regularly, though not everyone does. Far fewer people stay on top of the cooling system because modern systems need far less maintenance, often going up to 100k miles with no additional intervention.

This false sense of security can often lead to complete system neglect. That means that leaking pumps or degraded hoses are ignored, because the leaks go unnoticed. But cooling system failure can disable or destroy your engine just as running out of oil can.

Which brings us to oil, the important 'lifeblood' of your engine. It should go without saying that this is essential to engine reliability, but I'm shocked surprised by the number of vehicles that aren't that old that I see blowing the telltale blue smoke of oil combustion, their valve stem seals or piston rings worn beyond hope.

Keep your tires inflated, your coolant level up, and your oil reasonably fresh, and most cars will be reliable enough these days.

 

How to Get GREAT MPG: Driving Habits

No, I'm not saying you go get a Prius—that's cheating. Priuses (Prii?) are not only not as green as some think, they almost never pay for themselves in fuel savings, despite the awesome MPG.

I'm also not going to focus upon what you can buy to improve MPG. There are several products that can and will help, but nothing compares with driving behavior. Yes, I highly recommend a low-viscosity premium synthetic oil (like the fantastic new Pennzoil Platinum Pure Plus in 0W-20), but I'm going to focus on driving style.

The reason for this is that driving style more than all other factors explains the differences in fuel economy from otherwise identical vehicles. Large trucking fleet operators will tell you that the differences in MPG from one driver to another can be 30% using the same truck. Many fleets have incentive rewards that allow their best MPG drivers to drive faster (making more money per hour at the same rate per mile). After all, they can get better MPG at 65mph than another driver may get at 62mph.

It really boils down to two key ideas: how the engine makes power, and what happens to the energy it took to make that power after it as been used. Since the latter is the simplest, let's knock that out first.

Dude, Where's my Energy?

Stay off the brakes. Period. It's that simple. Every time you touch the brakes on a regular car, you are converting the kinetic energy of the car into wasted heat. Where did that energy come from? Hint: it's stored in a tank in your car, and you stop at stations to buy more at crazy high prices. Obviously, you must use the brakes to drive safely and obey the law. But you should absolutely take every measure to ensure you are only using them when needed. Kinetic energy goes up with the square of speed; if you double your speed, you have 4 times more kinetic energy. Three times faster? NINE times more energy. Stepping on the brakes to stop from 90mph will dissipate 81 times (!) the energy of stopping from a mere 10mph. That's energy you spent burning fuel and can never get back. Darn that 2^nd law of thermodynamics.

Staying off the brakes means coasting. It means recognizing red lights early, and letting less informed drivers pass you just in time to slam on the brakes for the red light you are coasting up to.

Here's a rule of thumb: try to only brake below 25mph. If you can keep your "brake engagement" speed low, your wasted energy will also be low—and the MPG will be much better.

Savvier readers are probably noting that I didn't mention anything about wind resistance or tire rolling resistance. It's simply the case that you cannot change either of these factors with driving style, and they do matter far less than the factors you do have some control over. Major on the majors, right?

Let's turn now to how the engine makes its power.

Power Preference

Your engine wants to make power a certain way in order to be as efficient as possible. Namely, it wants to be run at high load and low RPM conditions, near the point where it will explode in a violent cacophony of metallic carnage. Near—get it? Not AT. Price is Right rules are in effect—you go as close to the line as you can without going over, or you lose. You may have been taught not to "lug" the engine, that it would damage it. This is baloney.

The talented engineers of your non-GM automobile (and the cost reduction specialists who build GMs) do their best (GM: ok, maybe they could do better) to make sure you cannot physically load the engine beyond its mechanical limits. You simply won't be able to hurt the engine by putting too much load on it. Either the engine will stall or something else will give way, so worry not about hurting the engine. Lug away all you can, because it's the way your engine wants to be driven.

Explaining why takes a little geeking on automotive engineering, so bear with me.

Fuel is power. Air is needed to support combustion in appropriate ratios, but fuel is what makes power. To make the engine as efficient as possible, we need to ensure that the highest percentage possible of potential power (fuel flow into the engine) actually shows up at the transmission as available power.

Unfortunately, there are lots of other things going on in the engine that are trying to steal away the engine's power. The engine is sucking through a throttle, having to do a lot of work to pump air through the engine to support the fuel burning. The engine is driving an oil pump to keep itself lubricated—and a "Water" pump to keep it cool. These are called "pumping losses" because they represent lost power spent pumping air, "water" (coolant), and oil through the engine.

Another class of losses is called "parasitic losses" and they involve everything else that sucks power from the engine. This is the work to open and close the engine's valves. The work to turn the alternator and refrigerant compressors is also a parasitic loss. Frictional losses, like the friction of sliding the piston's rings up and down against the cylinder bore, are also considered parasitic losses.

Conveniently enough, there is a useful proxy that represents all the bad wastes of power in an engine: RPM. Every source of wasted power in an engine goes up with RPM.

Fuel-efficient driving then become a matter of regularly achieving the lowest RPM you can use for the conditions. Because engine RPM is affected so much by the transmission, we'll discuss that next.

Transmission Matters

A great transmission can really help fuel economy by ensuring that the engine is operating in its most fuel efficient condition under most driving. If you have a manual transmission, then it's easy to drive with economy; you just short-shift and keep the transmission in such a high gear that the engine seemingly can't accelerate. If you step on the accelerator and get a rush of acceleration instead of a whimper of protest, then you're doing it wrong.

Automatic transmissions are trickier to fool into giving you better MPG. There is a way that you can, though it requires some finesse. An important fact allows you to manipulate the transmission and fool it into giving you better MPG: the upshift and downshift points use different load factors.

Huh? Well think for second about how the transmission knows when to shift up or down. When it sees high throttle inputs, it holds the lower gears (because it thinks you want acceleration). If it sees a low throttle input, it assumes you are at a speed you want to maintain, or simply aren't accelerating as fast, so the lower gear isn't needed; it will upshift to the next gear.

Based on throttle inputs and programming about vehicle speed and engine speed ranges, the transmission will either upshift another gear, or downshift back to the lower gear. Low throttle=upshift to another gear. High throttle=downshift to lower gear (as for passing situations). The thresholds of where it will shift are vitally important to fuel economy.

Pay attention to this: the transmission will shift up to 3^rd gear (from 2^nd ) at a much lower throttle input than it takes to get it to shift down to 3^rd gear from 4^th. That means you can back off the throttle to cause the transmission to upshift, then add more throttle back without triggering the transmission to shift back down to the previous gear.

So to properly drive an automatic transmission for fuel economy, you should only add throttle as you go to higher gears. Perhaps 10% throttle until you get to 2^nd gear, then go to 20% throttle until it shifts to 3^rd gear, then 30%, and so forth. The resulting acceleration is faster in the higher gears than many people do, and slower in the lower gears. This is the key to better MPG with an automatic.

It will take some time to develop a feel for just how far you can push the throttle in a given gear without triggering a downshift.

Conclusions

Driving for fuel economy is about keeping RPM low and engine load high. Lower gears take load off the engine by letting it rev up—which is NOT what you want for MPG. Shift early, shift often. Keep the RPM down and throttle restrained. Then, stay off the brakes and coast wherever possible in lieu of braking. Practice is a bit and you can easily achieve 20% or greater improvements in MPG.

How to Choose a "Motor" Oil

Motor oil (which I actually prefer to call Engine Oil, since your car's source of motive power is more properly called an engine) is a confusing subject for many. Is there really a difference one to another?

Yes, there is-- and some differences matter, while others matter very little.

The single most important variable in an engine oil is viscosity, or how thick it is. This is the thing you should care about the most, and most people could pay attention to only this factor and be just fine with any national brand.

Related to the differences in oil thickness are how that oil changes thickness across temperatures. We all know that "molasses in January" is rather thick, but hot cooking oil can be almost water-like in consistency.

What a lot of people don't realize is just how non-linear is this behavior is. Notice on this graph that once you get the oil up to it's normal temperature (about 100C, the boiling point of water), the difference in thickness are too small to see on this Y-axis scale. You'd need to re-scale the graph to even note the difference.

The basic idea: cold oil is bad oil. The engine is designed to oil be a certain thickness (about 8-12 cSt), and any oil is too thick at even room temperature. Let's zoom in a little bit:

The chart above chose that even the thinnest oil (5w-30 here) on a sweltering hot day (40 degrees C is 104F) is still about SEVEN TIMES TOO THICK when the engine is "cold" on that sweltering summer day. So, if a "thin" oil on a hot summer day is 7x too thick, then you can be very comfortable assuming that a thick oil (like a 15w-40) is far worse on a cold day. For example, the first chart shows a 15w-40 oil on a cold winter start (32F) to be about 130x too thick! Now, this is at 32F, never mind what you frozen Midwesterners mights see on a -40F winter morning.

It's clear that the change in oil thickness with oil temperature is a bad thing. It turns out that some oils change thickness much less than others, even within the same "rating."

While all oils thicken as they cool, some thicken much less. Generally, synthetic oils excel in this regard.

Generally, the further apart are the numbers on the bottle, the less the oil thickens. There is a point where an oil gets so thick that it won't pour and it's essentially frozen solid. The colder that temperature, the thinner the oil is at temps warmer than that (and that's a good thing). For example, Rotella 15w-40 has a pour point of -30C, but Mobil 1 0w-30 is all the way down to -50C.

What about the resistance to heat? For most people, this isn't much of a factor. Oil temperatures in a properly running engine rarely, if ever, enter the danger zone. Not even in Death Valley in August. Most automotive cooling systems are sized to be able to let the air conditioning function in extreme heat. By comparison, keeping the engine oil cool is much easier and your oil holds a pretty steady temperature even on the hottest days.

The only real advantage of thicker oil is at extremely high oil temperatures that are a almost never going to happen in a modern, liquid-cooled car.

Next time you buy oil, just buy the thinnest stuff you can find in a nationally known brand (Valvoline, Mobil, Castrol, Havoline, Pennzoil, Quaker State, etc). If you do lots of cold starts in a climate that gets cold winters, buy the thinnest synthetic oil (mobil 1, Castrol Syntech, Royal Purple, etc) you can find.

Change the oil every 200 gallons of fuel you burn (for most cars that hold ~ 5 quarts) and don't worry about it anymore than that.