Driving Dissertation
Many years ago my Dad broke his leg. For several weeks, Mom had to drive to the grocery store. The family truck got 15 MPG with Dad driving, but 18 MPG with Mom driving!
Searching the internet for ways to improve fuel economy, the number 1 recommendation is always change your driving habits. It’s one thing to be told certain approaches improve fuel economy, but let’s look at why they work.
Years ago folks would install a vacuum gauge on their car. Usually you can lift off the throttle a little and not loose speed. Less throttle gives better fuel economy. When accelerating, the goal is to keep the highest vacuum while still speeding up. At least, try to keep it out of the high-power zone (low vacuum). Plymouth offered a Fuel Economy Indicator package in the mid-1970s that would light up a turn signal indicator when you were above a certain vacuum. The goal was to keep that light on. OBD-II (1996+) vehicles can use a Scan Gauge and view the MPG values directly from the ECU’s data stream.
Believe it or not, many fuel economy enhancement techniques work because they increase performance in the engine. This means you can maintain speed with less throttle. At any given AFR (specifically, the cruise target of 14.7:1), less throttle means less air and less fuel is being pumped through the engine.
The biggest enemy of fuel economy — and emissions — is what is called Transients. Old fashioned carburetors used an accelerator pump that shot extra fuel into the engine to cover lean spots when you punched the throttle; a rapid change in throttle angle that admits more air in a burst (a transient). Conversely, when you rapidly let off the throttle, the wet intake tract (intake manifold runners mostly) would be loaded with fuel. This extra fuel caused a temporary rich condition (another transient). Both waste fuel. Fuel injection systems also suffer from transients, even with electronic controls. A steady throttle state allows the fuel delivery system to reach equilibrium and maintain it. Smooth changes in throttle angle in either direction will help with fuel economy by reducing transient losses.
It takes more fuel to accelerate than it does to maintain speed. At the lowest level, it boils down to simple chemistry and physics; converting chemical energy to kinetic energy. Hard acceleration requires massive power. That kinetic power is derived from the chemical energy in the fuel; harder acceleration requires more fuel! I am amazed how drivers seem to want to race from one red light to the next. We all leave the next light at the same time when it turns green. Look ahead and if the light is green, accelerate at a normal pace. If it’s red, gradually accelerate, and maybe even coast most of the way. You’ll still have to put the brakes on and stop, but you waste less fuel. If you race from light to light, try to get a paycheck out of it. Race for $20 or pink slips (not recommended). If there’s no paycheck in it from winning, go for the guaranteed paycheck at the pump where it costs you less to fill up (recommended).
If you live in a hilly area, elevation changes frequently. Sometimes you’re driving up hill, then a bit later, down hill. (Cruise control will work hard to maintain a constant speed all the time.) As you near the bottom of a hill, accelerate a bit faster. As you climb the next hill, allow the vehicle to slow down a bit. A 5-10 MPH swing may not impede traffic, but can gain you an extra 5% to 15% better fuel economy. The goal is to reduce throttle angle as much as possible, and stay out of the “Power Zone”.
In the old days, carburetors used a choke to enrich the AFR until the engine warmed up. Modern fuel injected engines do the same thing, except it’s using Compensation Factors and electronic controls. Until the engine warms up, the fuel injectors dump extra fuel as a percentage of total air intake.
A test vehicle was started at ambient freeze point; 32 degrees F. The Injector Pulse Width (IPW) was monitored as the engine warmed up at an idle. The IPW started at over 30 ms. After about 2 minutes it had dropped to around 8 ms. At full operating temperature, it stabilized at around 2 ms. As a percentage, the cold engine required 15X more fuel than the same engine warmed up!
Cold engines are less capable of vaporizing the fuel. Since only fuel vapors burn, it takes more total fuel to get the needed fuel vapors to run the engine. This means hard acceleration on a cold engine wastes way more fuel than when it’s at operating temperature — by a percentage of throttle angle & load! Until the engine warms up, baby the throttle.
With a carbureted engine, I would pop the transmission into neutral while going down hills to improve fuel economy. In gear, higher engine speeds pulled more fuel through the carburetor's idle circuit, even when I wasn’t requesting power. First, that is illegal (I failed my driving test back in the 1980s doing that!). Secondly, modern vehicles use a decel fuel cut strategy that turns the injectors off when decelerating, or going down a hill with no throttle; thus using no fuel.
Which brings me to Hypermiling. The concept is to accelerate to speed, put the transmission in neutral and shut the engine off. When you have dropped about 15 MPH, start the engine, accelerate to speed again, and shut the engine off... rinse and repeat.
First, as stated before, this is illegal! Secondly, it requires immense concentration, which could distract attention from pedestrians and other traffic hazards. Thirdly, power brakes, steering, and even the alternator are not working when the engine is off. For many reasons, Hypermiling is considered unsafe. However, mildly applied, some of the techniques can be practically and safely implemented to improve fuel economy.
As somewhat of a side note, vehicle weight affects fuel economy. Heavier vehicles require more power to accelerate. Take a look in your trunk. Are you lugging around 150 pounds of useless weight? Just cleaning out un-needed junk from the vehicle can improve fuel economy. Some guys take this concept to extremes. They remove sound insulation, unnecessary accessories, the back seat, even drill holes in hidden sheet metal to shave extra ounces.
Think about this; Jr. gets 20 MPG, but Granny gets 26 MPG in the same car! How you drive affects your fuel economy.
FE1
MPGenie Basics 051 Training - Driving Dissertations
