Fuel and Engine Types

Automotive fuels come in a variety of forms and characteristics - in liquid or gaseous state - but all have one primary function : to provide motive power to an internal combustion engine. The internal combustion engine (ICE) are of two(2) main types, namely:  (a) a spark-ignited engine, and;   (b) compression-ignited engine.

    There are of course other types of engines, though mainly on the experimental stage of develop- ment -  such as the battery-powered car (which  actually runs on a series of electric motors),   the solar-powered vehicle, and an experimental model that is based on fuel-cell (hydrogen gas)  technology.  However,   the commercial application of these protypes and their wide-spread use appear to be still far off into the future.

    The I.C. engine type is in turn fueled differently. For instance, spark-ignited engines are fed a mixture

of gasoline and air in vapor form thru the intake manifold and then ignited by a "spark plug," a device that electrically fires off thousands of volts of energy at regular intervals.

    Compression-ignited engines, on the other hand (also known as "diesels," named after its inventor, Ru-dolph Diesel,  some 100 years ago),  depend upon the heat of compression to do the trick,   i.e., ignite the fuel under tremendous temperatures and pressures. Appropriately enough, the fuel used is called diesel fuel (however, some people insist on calling it "diesoline" or "crude"  which can be very confusing).

    Sometimes, a mixture of LPG (light petroleum gas) or CNG (compressed natural gas) is used as an "alternative fuel," for compression-ignited engines, but this topic  has pros and cons that won't be dis-cussed here.  So, too, the so-called "steam engine" invented by James Watt but no longer used in the modern automobile.

    Both gasoline and diesel fuel are fossil fuels, meaning they are derived from crude oil which is mined/ drilled underneath the ground (or sea) as remains of fossil deposits over millions of years ago. Both fuels are made up of hydrogen (H) and carbon (C) atoms arranged in varying complexity and are therefore termed "hydro-carbon fuels" or epetroleum fuels.  below are discussed the characteris- tics of these fuels. Let's take gaso-line first as a fuel for an overwhelming number of light vehicles like passenger cars.

A.  GASOLINE

    Gasoline - whether leaded or lead-free (i.e., unleaded) - is a liquid, clear-brown color, highly-flammable fuel that has a low flash point (ignitability), but burns rapidly (within 0.010 seconds) at high temperatures (around 850 deg. F). The green color of that unleaded gasoline at your favorite service station is nothing but artificial coloring to distinguish it from premium leaded and other types of fuels. Your typical gasoline is a mixture of aromatic hydro- carbon compounds such as benzene, tol- uene, and xylene,     plus aliphatic HC compounds. Additives are also added and function as detergents (cleaners/scrubbers), engine valve lubricants & coolants, and as octane enhancers.     The last part - octane - determines the gasoline's grade.     A modern, premium unleaded 91-octane gasoline typically shows the following  ingredient specifications.

    Leaded or Unleaded? Premium or Regular?

    The average car owner wants to put only the best gasoline in his or her new automobile, but what grade is really the best? - this is the most common comment one hears around. Most mechanics and service station attendants will tell you to fill up on the 'best-quality' gasoline available (also, the most expensive from their pumps). Others will spread the lie that "using unleaded gas is bad for the engine; use only premium."    Un-fortunately, the oil companies are not helping the situation any by keeping mum on the subject and yet spen-ding millions on advertisements extolling the merits of their respective brands as    'acceleration boosters,' 'super-formula,' 'power fuel', etc.     What's the truth to these claims?

    Tetraethyl lead first saw use in the 1920s when automotive engineers were seeking a solution to the prob-lem of "engine knock," otherwise known as premature detonation. Ordinarily, the volatile mixture of air and gasoline would uniformly burn when the spark plug ignites the mixture as the engine piston is almost at  'top dead center' (topmost portion of the cylinder), giving the piston a tremendous 'push' and driving it all the way down and thereby imparting a rotational action to a crankshaft.  The mechanical energy imparted by the pis-ton is felt at the driving wheels, making the car go forward (or, backward).

That Pesky Engine "Knock" and Gasoline Octane Rating

    Pockets of air-gasoline mixture in the cylinder may, however, burn ahead (or, pre-ignite) of the rest of the mixture, prematurely pushing the  piston down on its upward motion. The resultant push-shove is felt   as a vibration or knock, sometimes severe enough  to damage the engine. The least expensive solution : add lead to gasoline. Lead has that desirable quality of resisting gasoline's ignition or burning -  a measure of  its so-called Octane  rating or number. The higher the octane number, the greater is the resistance to knock or detonate. So, ordinarily one should get the highest octane gasoline for one's car, right?  Wrong!

    Gasoline types are graded by varying  octane numbers, the numbers depending upon the country you're in. In the United States, the following are the accepted classifications in 1998:
 

     Octane numbers could also go as high as 100 to 104 although the  use of these high-octane gasoline is gen-erally limited to racing cars and other specialized, "non-street" vehicles. In deciding  what gasoline grade to use, one should remember two(2) things:   first, low-octane fuel is detrimental to modern, high-compression, automobile engines because of engine knock or detonation;   second, using a high-octane gasoline will not necessarily improve the engine's performance. The best way to determine which octane is suitable to your engine is by consulting the car manufacturer's manual. If that isn't available for you, try the trial-and-error method (with the help of a mechanic): try advancing/retarding the ignition timing setting    (by loosening the distributor hold-down screw and turning it around). While doing this, fill up with the different types of gasoline available from the service station. Once you no longer feel the knock at various speeds on a roadtest, as well as "feel" the maximum power that the car can give, that's the "right" fuel for you. Of course, you have to re-member that given Metro Manila's slow traffic, you probably don't need that power and acceleration, so one can settle for a lesser octane and save,  too.  Nowadays, local unleaded fuel is between 89-95 octane.   The "super" or "special variety" is more expensive because of those additives that I mentioned.    Anyhow, it's safe to assume that the quality of our fuel is inferior to that in the U.S.

    Oil firms in the Philippines, unfortunately, do not publish octane ratings of their gasoline products. Neither do they categorically state that their "Premium" gasoline is still leaded   (waiting perhaps for the 18-month grace period given by the 1999 Clean Air Act before leaded gasoline is banned, finally!). (Postcript: Shell and Seaoil now announce the Octane No. in their service stations. Also, leaded gasoline is no longer sold- IJR). Me, I have been using unleaded gasoline since it was first introduced locally in late '93, and have yet to expe-rience any problem in  the cars I have used - an '89 carburetor-type, and a '94 fuel-injected (ECCS) type. So, why do so many well-meaning car owners continue to use leaded fuel? The answer probably lies in fear of the unknown.

    Valve Seat Damage and Countermeasures

     Lead had a nice side-effect, too, aside from preventing knock :  it cooled and lubricated the engine valves by coating the seats with lead oxide. The use of lead as a cooling agent was a cheap compromise to a major engineering headache encountered by the early researchers.

    Still, lead poisoning thru vehicle fumes couldn't be ignored. With strict legislation defining allowable emis-sion standards set by the U.S. in the 70s, tremendous strides have been accomplished in the areas of engine design, pollution control, fuel delivery systems, and fuel technology in the industrialized world. Unleaded ga-soline, for instance, now contains almost no lead (0.15 gms./liter or less) but have other octane enhancers like MTBE (methyl-tertiary-butyl-ether). MTBE also oxygenates gasoline and reduces CO pollultion.     Modern engines now have aluminum heads, with hardened steel valve inserts to handle the high temperatures.  The use of on-board computers make it possible  to have precise mixtures of fuel inside the cylinder, eliminating most of the knock. A lean mixture, plus afterburners or,  catalytic converters, enable the engine to run clea-ner.

    So, if you're driving a circa-90 model, you have nothing to fear from unleaded gasoline. For those driving around in their '60, '70 (and some '80) models, check with the manufacturer. If only leaded gasoline is for you, don't despair yet. Have hardened steel valve seats installed next time, and that should solve your problem. There are also a few imported lead additives on the after-market, but since leaded gasoline will still be plent-iful for the next 18 months, so what's the point of buying? Inevitably, though, you'll probably have to get rid of your vintage cars. But come to think of it, you'll be doing the rest of this country and us a BIG FAVOR.

B.  DIESEL FUEL

    Diesel fuel burns more efficiently than gasoline and produces more energy (BTU) per unit time. It ignites

at a higher temperature (55 deg. C) than gasoline (which ignites even at sub-zero temperature), though the combustion rate is much slower. Because it  is less volatile and more viscous than gasoline, it must be comp-ressed to a high pressure and temperature (450-600 psi) to ignite it inside the combustion chamber. Typically, as the piston goes up and approaches top dead center, a spray of diesel fuel is sprayed under pressure from the injection pump via an 'injector'. The vaporized fuel is mixed with air introduced thru an intake valve, and the mixture spontaneously burns, driving back the piston and producing power to run the vehicle.  Fuel specifications for diesel fuel are found here .

    Indirect  vs. Direct Injection and Fuel Knock

    As mentioned above, fuel is introduced into the cylinder by either: (a) thru a 'pre-combustion chamber'; or, (b) directly into the cylinder. The first type is known as an Indirect Injection Engine (IDE) while the second type is know as a Direct Injection Engine (DI). In the early days of the diesel, fuel was always sprayed into a pre-combustion chamber where it was allowed to thoroughly mix with the air pior to combustion. Fuels in those days were then relatively 'coarse' and would knock terribly. The sound was known as a 'diesel knock' and was an accepted fact for a diesel. That's why early engines had to be built robustly, even massively, to handle the vibration.

    But developments in engine design, fuel technology, and emission restrictions have altered the diesel. With a Cetane (octane number for diesel fuel) Index now at 48 up, diesel fuels have less tendency to knock. Too, the modern diesel is even more high-compression.  Fuel now could be introduced   directly into the cylinder, with the piston  having a recessed head where combustion could take place. The DI engine is  usually big-bore with a shorter stroke (hence the engine is more compact).

    Increasing the Cetane Index results in :

            1.  More power output and fuel efficiency;
            2.  Low-temperature startability;
            3.  Less noise; and
            4.  Less harmful emissions.

    The power of the diesel is such that it produces more torque (torque can be defined as  the ability to over-come inertia) for less horsepower than a gasoline engine.   Most trucks and heavy equipment therefore are diesels, since they move around large quantities of material and people at lesser cost (diesel fuel is cheaper to produce than gasoline). A few automobile companies like Mercedes-Benz and Isuzu have traditionally built diesel cars for a loyal market.

    More Diesels on the Road?

    So, with the inherent advantage of a diesel in terms of fuel efficiency and lower fuel cost, why isn't there more demand for diesel units?

    Again, the answer lies in aesthetics, performance, and economics - or a perception thereof by the typical vehicle owner.

    Firstly, a diesel engine is noisy compared to a gasoline engine, despite improvements  in  the octane rating of diesel fuel, the introduction of electronic governors for injection pumps, and the common rail technology of fuel distribution.  Performance-wise, the diesel can hardly compete with a gasoline car in acceleration,   top speeds, and other features dear to the heart of a 'performance driver.' Typically, too, a diesel engine, when it breaks down, is more expensive to repair than gasoline engines because the parts cost more.

    SUVs - sports utility vehicles - are a new generation type of diesels that appear to get the consumers' at-tention lately. A cross between a car and a utililty vehicle, SUVs are the fastest-selling units in many parts of the world today. Whether customers' interest on these vehicles will be sustained cannot as yet be determined at this time (Note: SUVs still enjoy rising popularity going into the 2000s - IJR)..

What's Ahead for Automotive Fuels?

    The "war" against air pollution caused by mobile sources (read: cars, trucks, buses, jeepney, etc.)  is far from over. The gains thus far have been significant for those countries which have placed the highest priority to combat pollution thru stringent legislation,  new engine technology and controls, and significant changes in fuel infrastructure. The West and Japan have taken the lead in these new technologies, in enacting enabling laws and regulations that have slowly but inexorably tightened emission standards over the years, and in sec-uring the cooperation of the citizens by their governments.

        Renewed Interest in Diesels/Concerns

    Renewed interest in diesel stems from its potential to improve passenger-vehicle fuel economy.   Today's best diesel passenger vehicles are more efficient than their gasoline counterparts, reducing carbon emissions by roughly 30 percent. However, diesels emit twice as much nitrogen oxides as gasoline vehicles and   10 to 100 times more particulate matter, even as gasoline engines produce more hydrocarbon, CO, and volatile or-ganic compounds (VOC). In many parts of the world, lead is no longer used in gasoline as additive. The use of catalytic converters and particulate traps have, however, greatly abetted exhaust gas emissions.    A visual view of these sample catalysts and filters is given here.

    The concern lies with the increasing production by manufacturers of light- and  heavy-duty diesels in the years to come since these are the main source of diesel pollution.  For instance, with  approximately 40 mil-lion diesels currently running on highways in the U.S., its        Environmental Protection Agency (EPA) has drawn up a master plan to progressively lower emissions in 1998, 2004, and 2010. Of the five(5)diesel fuel parame-ters studied by EPA,  lowering the sulfur content of diesel fuel appears to be the most promising  (see below).