From the Lair of the Liar; September 2010 --
The other day,
on another BLOG, the Great Grumpy Z described steps he undertook to improve the gas mileage on his "Red Rocket" by replacing all six spark plugs. Because there are so many other factors which could also affect mileage, this POST will attempt to address those of which Grumpy is aware. Since he received a Bachelor of Science degree in Mechanical Engineering, and has since childhood held a fascination and interest in engines and other mechanical devices, Grumpy feels well-qualified to bloviate at some length on this subject. Readers for whom such discussion would be boring (they already know all about the subject) or who have no particular interest in improving their gas mileage (they have money -- and gasoline -- to "burn"), should stop at this point and find something better or more interesting to do with their time.
For the rest of us, let us begin with the effect of spark plugs on gasoline mileage. The spark plugs provide the ignition source by which the combustible mixture in the cylinder is ignited at the proper time. The proper time is a mysteriously variable point, where the piston is at the optimum distance during its compression stroke such that the process that follows, namely the initial ignition and subsequent burning of the combustible mixture (no, the mixture does not "explode" -- instead the burning progresses like a flame front out in a more or less radial direction from the spark plug, eventually reaching the most distant part of the cylinder) and the further travel of the piston through the top of its travel and continuing downward in the power stroke, will extract the maximum amount of available energy from the combustion process. If this sounds complicated, well it is!
The fact is, this "mysteriously variable point" varies with speed of the engine, number of spark plugs per cylinder*, and some details of the engine configuration whose effect can only be determined after the engine is designed and tested. Only then can the optimum timing of the ignition be determined for each given engine speed. Now, there are details of the spark plug which have small, but significant, effects on ignition timing. One of the most important details is the gap between the electrodes of the plug. For automobiles built in the 1920's up to the 1950's, the "optimum" gap was about .028 inches. A gap as wide as .030 would degrade gasoline mileage by as much as 20%. Modern automobile engines, which typically have an ignition system of 50,000-75,000 volts to generate a spark, use a much wider gap: .038 to .044 inches. The electrodes on the "old time" plugs was made of Tungsten or Copper-Tungsten alloy. These metals would erode due to the electrons in the spark carrying away particles of the electrodes which are exposed to high temperatures; sufficient wear on these plugs necessitated regapping or replacing in 20-25,000 miles. Most modern spark plugs used today have Platinum or Iridium at the points where the electric arc occurs; these materials are more resistant to the erosion under the high temperature conditions, so they may be expected to give satisfactory operation for 100,000 miles or more.
Now let us talk about another factor which can affect gasoline mileage: the difference in the amount of energy that can be extracted by combustion of different "gasoline". I have placed quotation marks around the word "gasoline" because the fuel we call gasoline that we put in our automobile tanks these days is not the same as it used to be. Pure gasoline -- what Great Grumpy used in his 1928 Chevrolet -- contains about 122,000 BTU's per gallon and it burns at a fairly rapid rate. Most of the gasoline we can buy today contains 10% Ethanol (a form of alcohol). The BTU per gallon is not much different -- slightly lower than pure gasoline -- but the big difference is that the Ethanol slows the rate of burning, so much that the ignition timing should occur earlier than what is optimum for pure gasoline. The engine manufacturer is trapped with the requirement to design his engine to use pure gasoline, but also to be able to use the 10% Ethanol blend. The result is that when the 10% Ethanol blend is used, gasoline mileage suffers. Interestingly, the penalty Great Grumpy has observed is about 10%! This has been my observation, well documented on the '86 Ford F-250 pulling a travel trailer, but also strongly indicated in freeway driving cross-country with the Red Rocket. Perhaps someday a "smarter" engine computer will be able to sense the quality of the fuel in the tank and readjust the timing toward optimum so that this penalty will disappear.
In Brazil, where service stations offer both pure gasoline and pure Ethanol, the car buyer has a choice between an automobile designed to utilize Ethanol, or gasoline. An engine designed to use pure Ethanol can take advantage of Ethanol's higher "Octane" rating by having a much higher compression ratio (12-15 to one), thereby increasing the burn rate of the air-fuel mixture. Such an engine, using pure Ethanol, will get much superior mileage (and more power!). The higher compression ratio results in a significantly-increased engine efficiency compared to an equivalent-sized engine designed to use pure gasoline.
Several factors that are mostly within the driver's power to control, also have a strong effect on gasoline mileage. These include: Speed of travel; Tire design and inflation; Total weight of vehicle and contents; Wind direction and velocity; Driving habits, particularly in stop and go traffic; Vehicle maintenance.
Speed of travel is usually touted as a major factor in affecting mileage. However, the Great Grumpy has found that, with the Red Rocket (and he believes this would be true of other modern automobiles) cruising at a steady speed at 55 miles per hour does not result in appreciably improved gasoline mileage compared to cruising at 65 miles per hour. In fact, the slower speed may actually result in worse mileage, especially if you are driving in hilly terrain. This is because, at the slower speed the engine will be required to shift into a lower gear when going up hills; at the higher speed, the momentum of the car will carry it up many hills without the need to down-shift. To carry this idea further, it must be pointed out that modern cars are designed with an overall gear ratio (engine speed per vehicle speed) such that there is an optimum vehicle velocity at which maximum mileage is achieved on a straight and level highway. This is one of the reasons why advertised mileage shows higher values for highway (or freeway) driving and lower values for city driving.
The reason higher speeds are detrimental to gasoline mileage is because of wind drag. Improved aerodynamics of modern automobiles have done much to decrease the wind drag caused by turbulence that plagued the "old cars" (take the Great Grumpy's "squarish" 1928 Chevrolet, for example!). But, regardless of the "sleekness" of the body design, the fact remains that a certain mass of air must be displaced to accommodate the volume of the vehicle travelling through it. That effect is closely tied to the frontal cross-section of the vehicle; that is, the area and shape of a hole through which the vehicle could pass without any clearance. The drag induced by this effect increases as the square of the airspeed; thus if you are driving into a wind whose velocity is 25 miles per hour, and your vehicle is travelling at 75 miles per hour, the wind drag will be four times as much as with no wind and your vehicle travelling at 50 miles per hour. In fact, this wind drag effect is what limits the top speed of a normal passenger automobile. The power to overcome this wind drag becomes equal to the horsepower output of the engine.
Tire inflation -- and tire carcass design -- affect the rolling resistance of the wheels. Higher inflation pressures -- up to the maximum specified by the tire manufacturer -- will help increase gasoline mileage, but only by a percentage point or two. Road noise will be increased, the ride will be harsher and higher tire inflation is not recommended except when travelling freeways and other paved highways in excellent condition, or when operating the vehicle at or near its gross load limit
Tire carcass design is best illustrated by a comparison of radial tires and bias-ply tires. In the "olden times", only bias-ply tires were available for use on automobiles. The rolling friction of bias-ply tires is greater than radial tires. This causes heat build-up in the bias-ply tires to a much greater extent than in radials, hence the radial tires are less prone to failure due to becoming overheated. [However, either tire design will become overheated if it is driven under-inflated, overloaded or driven at ultra high speeds.] Radial tires provide a bonus by improving the handling quality of an automobile (also true when used on a towed vehicle).
The total weight of the vehicle, including its contents, affects gasoline mileage primarily because of the effect on rolling resistance of the tires. Thus, as mentioned above, increasing the tire inflation up to the maximum allowed by the tire manufacturer will reduce the effect of weight on rolling resistance.
If travel involves going through hilly terrain, or at varying speeds, then weight will also affect gasoline mileage. This situation is somewhat analogous to city driving where traffic causes lots of stop-and-go operations. Hilly terrain, stop-and-go, or other routes that involve varying speeds, may be avoided or at least minimized by carefully choosing the route by which you travel.
Wind direction and velocity effects have been explained in the discussion above regarding vehicle velocity. The driver's choice, when driving into a head wind, is to drive at a somewhat slower speed -- or delay travel for a few hours or a day or so, expecting less unfavorable winds. Winds are generally lighter at night and early morning; winds generally increase in velocity in the mid- to late-afternoon.
Vehicle maintenance is the last subject I have listed that affects gasoline mileage. Assuring that tires are properly inflated, is part of maintenance. Another item is maintaining the proper level of the recommended engine oil; overfilling the crankcase results in the crankshaft "splashing" through the surface of the oil each revolution. This not only creates internal engine drag but also damages the oil by "frothing" it (introducing air into the oil) and heating the oil above what would be normal. Use of the oil grade and viscosity rating recommended by the engine manufacturer is extremely important, not only for good gasoline mileage, but also for achieving maximum engine life. While higher viscosity oils tend to assure better lubrication, modern automobile engines last a very long time, using very low viscosity oils. If your owner's manual recommends SAE 5-20 (a viscosity of 5 at low temperatures and a viscosity of 20 at normal engine operating temperatures), then do not assume you will be better off going to SAE 10-30; you could actually cause damage to your engine, especially during a cold start and initial operation until normal temperatures are reached. The more viscous oil will not lubricate as well as the recommended lower viscosity oil under these low temperature conditions.
A most significant item of maintenance for assuring good gasoline mileage is to replace worn piston rings and valve guides and seals when cylinder pressure readings indicate wear is beyond the manufacturer's specifications. An indication of excessive wear in these areas is an oily smoke emitted from the exhaust pipe, particularly after decelerating down a long hill, or standing with the engine idling for a few minutes at a stop light, and then accelerating up to speed. A blue cloud in your rear-view mirror is a message that your engine may be due for a major overhaul. NOTE: If this symptom exists, your car will not pass the emission test!
In the "olden days", automobiles used carburetors to meter and aerate the gasoline as it mixed with the intake air. This means of metering the proper amount of fuel to be mixed with the intake air was imprecise at best and the best gasoline mileage in those days was less than some of the so-called gas-guzzlers of today. Modern automobiles use "fuel injectors" to inject a much more precise amount of fuel, computed to be the right amount according to air temperature, air density (altitude), engine speed and engine power demands. Notice the word "computed" in the previous sentence! Yes, modern automobiles have an engine control computer -- in fact, there may be several computers doing various tasks in the modern automobile. The engine control computer receives electronic signals from sensors that measure the parameters detailed above. Engine power demand is derived from a sensor on the accelerator pedal (or cruise control computer). Other signals used by the engine control computer include crankshaft angle (or piston position relative to the top of it compression stroke), free Oxygen content in the exhaust gases, exhaust gas temperature (a high exhaust gas temperature indicates that the optimum air-fuel ratio has been achieved), as well as a number of engine parameters related to engine health.
Fuel injectors can malfunction for various reasons. In the "Olden Days", an engine miss was usually attributable to a bad spark plug. In today's modern engines, an engine miss is more likely to be caused by a malfunctioning fuel injector. A sticky fuel injector will not open when needed; this problem may be able to be solved by using a good fuel-injector cleaner (introduced into the fuel tank when fuel level is fairly low). A worn fuel injector can cause excessive fuel consumption because it fails to shut off completely. This problem is significant only during idle and low power states where the injector should be closed most of the time. [Uncle Bob has this problem on his 1990's era Cadillac; the cost of time and labor to trouble-shoot and determine which of the eight injectors is faulty was deemed to be more expensive than the cost of the extra gasoline consumed during those phases of operation where the leaky injector caused excessive fuel flow.]
My last in the items in a modern automobile that can go wrong is a computer! The engine computer was just mentioned; if it has a failure to sense one of the parameters needed to properly compute the timing for ignition, or the length of time for a fuel injector to open, then it obviously cannot effect the best gasoline mileage of which the engine is capable when everything is functioning. The transmission computer can fail in a similar manner: not being able to sense engine or vehicle speed. A typical way in which modern electronics will fail is a mechanical failure of an electronic connection: Cold solder joint; broken "leg" of an electronic component; broken (fatigued) printed circuit trace; incomplete connector insertion (including poorly-designed, i.e., mismatched male-female connectors); etc. Grumpy has seen and/or experienced each of these -- on commercial aircraft as well as on one of his personal automobiles. Electronics in general, and computers in particular, can be extremely reliable if designed for the environment in which they are supposed to function, and if they are programmed and assembled in a workmanship-like manner.
*Aircraft piston engines (and the 1928 Nash Touring car) have two spark plugs in each cylinder.
