Petrol and Driving Performance

This article was first published in 2001.

When most drivers think about petrol, it is to remember to fill up and maybe to check the price. Because petrol almost always performs well, drivers forget what a sophisticated product it is. More thought would reveal a demanding set of performance expectations:

• An engine which starts easily when cold, warms up rapidly and runs smoothly under all conditions.
• An engine which delivers adequate power without knocking.
• A vehicle which provides good fuel economy and generates low emissions.
• Petrol which does not add to engine deposits or contaminate or corrode the fuel system.

While proper vehicle design and maintenance are necessary, petrol plays an important role in meeting these expectations.

This article discusses how petrol's characteristics affect driving performance.

Volatility

Driveability is the term used to describe how an engine starts, warms up and runs. It is the assessment of a vehicle's response to the accelerator, relative to what the driver expects. Driveability problems include: hard starting, backfiring, rough idle, poor throttle response and stalling (at idle, under load or when decelerating).

The key petrol characteristic for good driveability is volatility - the petrol's tendency to vaporize. Volatility is important because liquids and solids don't burn; only vapours burn. When a liquid appears to be burning, actually it is the invisible vapour above the surface that is burning. This rule holds true in the combustion chamber of an engine; petrol must be vaporized before it can burn. In cold weather, petrol is blended to vaporize easily. This allows an engine to start quickly and run smoothly until it is warm. In warm weather, petrol is blended to vaporize less easily to prevent vapour lock and minimize evaporation, which contributes to air pollution. It is important to note that there is no single best volatility for petrol. Volatility must be adjusted for the altitude and seasonal temperature of the location where the petrol will be used.

Three properties are used to measure petrol volatility: vapour pressure, distillation profile and vapour-liquid ratio. A fourth property, driveability index, is calculated from the distillation profile.

1. Vapour Pressure

Vapour pressure is the single most important property for cold-start and warmup driveability. (Cold-start means that the engine is at ambient temperature, not that the ambient temperature is cold.) When the petrol's vapour pressure is low, the engine may have to be cranked a long time before it starts. When it is extremely low, the engine may not start at all. Engines with port fuel injection appear to start more readily with low vapour pressure fuel than carburetted engines.

2. Distillation Profile

Petrol is a mixture of hundreds of hydrocarbons, many of which have different boiling points. Thus petrol boils or distills over a range of temperatures, unlike a pure compound - water for instance - that boils at a single temperature. A petrol's distillation profile is the set of increasing temperatures at which it evaporates for a fixed series of increasing volume percentages - 5%, 10%, 20%, 30%, etc. - under specific conditions. (Alternatively, it may be the set of increasing evaporation volume percents for a fixed series of increasing temperatures.) The diagram here shows the distillation profiles of average summer and winter petrols.

Various ranges of a distillation profile have been correlated with specific aspects of petrol performance.

Front-end volatility is adjusted to provide:

• easy cold starting
• easy hot starting
• freedom from vapour lock
• low evaporation and running-loss emissions

Midrange volatility is adjusted to provide:

• rapid warmup and smooth running
• good short-trip fuel economy
• good power and acceleration
• protection against carburettor icing and hot stalling

Tail-end volatility is adjusted to provide:

• good fuel economy after engine warmup
• freedom from engine deposits
• minimal fuel dilution of crankcase oil
• minimal hydrocarbon exhaust emissions

The diagram here graphically illustrates these correlations. The temperature range is approximate; the exact range depends on the conditions that exist in the location where the vehicle is driven.

3. Vapour-Liquid Ratio

The vapour locking tendency of a petrol is influenced both by the temperatures at the front end of its distillation profile and by its vapour pressure. But the property that correlates best with vapour lock is the temperature at which the petrol forms a vapour-liquid ratio of twenty (V/L=20) - the temperature at which it exists as twenty volumes of vapour in equilibrium with one volume of liquid at atmospheric pressure. This correlation was developed for vehicles with suction-type fuel pumps and carburettors. How well it applies to later-model fuel-injected cars with pressurized fuel systems is not known.

Driveability Index

While each range of the distillation profile is important, the petrol represented by the entire profile is what the engine must distribute, vaporize and burn. To predict cold start and warmup driveability, a driveability index (DI) has been developed using the temperatures for the evaporated percentages of 10% (T₁₀), 50% (T₅₀) and 90% (T₉₀):

DI= 1.5(T₁₀) + 3.0(T₅₀) + (T₉₀)

The DI varies with petrol grade and season; the normal range is 850 to 1300. Lower values of DI generally result in better cold-start and warm-up performance, but once good driveability is achieved, there is no benefit to further lowering the DI.

Volatility Specifications

The petrol specification, ASTM D 4814, controls the volatility of petrol by setting limits for the vapour pressure, distillation profile and vapour-liquid ratio properties. The specification employs six vapour pressure/distillation profile classes (see table) and six vapour-liquid ratio classes. The specification assigns one vapour pressure/distillation profile class and one vapour-liquid ratio class each month to each geographical area (state or portion of a state) in the United States based on altitude and the expected ambient temperature range. ASTM D 4814 does not include a DI requirement. Several organizations have proposed various DI limits, but none had been approved at the time of writing.

Petrol volatility not only affects a vehicle's driveability, but also its hydrocarbon emissions - both evaporative and exhaust emissions. Because of this relationship, the US federal government and some states limit petrol volatility to control the aspect of air quality affected by hydrocarbon emissions. ASTM incorporates federal volatility regulations into the petrol specification as they are promulgated. Fluctuating volatility requirements make petrol manufacture and distribution a complex process. A refiner producing petrol for a multi-state area may have to make petrols with several different volatilities and change the volatility from month-to-month. And each petrol has to be kept separate while it is shipped to the appropriate location.

Antiknock Performance

Knock-free engine performance is as important as good driveability. Octane number is a measure of a petrol's antiknock performance - its ability to resist knocking as it burns in the combustion chamber. There are two laboratory test methods to measure the octane number of a petrol. One yields the Research Octane Number (RON), the other, the Motor Octane Number (MON). RON correlates best with low speed, mild-knocking conditions; MON correlates best with high-speed and high-temperature knocking conditions and with part-throttle operation. For a given petrol, RON is always greater than MON. The difference between the two is called the sensitivity of the petrol.

Because RON and MON are measured in a single cylinder laboratory engine, they do not completely predict antiknock performance in multicylinder engines. There is a procedure to measure the antiknock performance of a petrol in a vehicle. The resulting value is called Road Octane Number (RdON). Since vehicle testing is more involved than laboratory testing, there have been a number of attempts to predict RdON from RON and MON. The equations take the form:

RdON = a(RON) + b(MON) + c

A good approximation for RdON sets a=b=0.5 and c=0, yielding (RON + MON)/2, commonly abbreviated (R+M)/2. This is called the Antiknock Index (AKI). The US Federal Trade Commission requires dispensing pumps in the US to be labelled with the petrol's AKI. (The petrol being dispensed must have an antiknock index equal to or greater than the posted value.) Owner's manuals also must indicate the octane requirement of vehicles by AKI. (Older owner's manuals of some foreign cars specify RON; some more recent ones specify both RON and AKI.)

Neither the AKI nor the several other single-value indices that have been developed work for all vehicles. The performance of some vehicles correlates better with RON or MON alone than with a combination of the two. And for a given vehicle, the correlation can vary with driving conditions. As the formula indicates, petrols with the same AKI can have different RONs and MONs. This may explain why a vehicle knocks with some fill ups of the same brand but not with others; or why it knocks with one brand of petrol but not with another. Of course, for a comparison to be valid, the vehicle must be operated under identical conditions, which is not easy for the typical driver.

Since 1984, vehicles have been equipped with more sophisticated control systems, including sensors to measure, and engine management computers to adjust for, changes in air temperature and barometric pressure. These vehicles are designed to have the same AKI requirement at all elevations and the owner's manuals specify the same AKI petrol at all elevations.

It is difficult for a driver to know whether a petrol has the antiknock performance the engine requires when the engine is equipped with a knock sensor system. These systems, which temporarily retard spark timing to eliminate knocking, are installed on many late-model engines. Retarding the spark reduces power and acceleration. The knock sensor responds so quickly that the driver never notices the knock. Loss of power and acceleration will be the only clues that the antiknock quality of the petrol does not meet the vehicle's octane requirement.

Using petrol with an antiknock rating higher than that required to prevent knock or to prevent spark retardation by the knock sensor will not improve a vehicle's performance.

Power

The power an engine develops depends on its design. In general, the more air an engine can process, the more power it can produce. Major design considerations for power are the displacement of the engine, the compression ratio, and the presence of a supercharger or turbocharger. Other factors affecting power are the number of valves per cylinder, valve timing, and spark timing. Because different grades of petrol have essentially the same heating value, they all provide the same power as long as their antiknock performance meets the engine's requirement.

Fuel Economy

Fuel economy is usually expressed as the number of miles travelled on one gallon of petrol, miles per gallon (mpg), or litres consumed per 100 kilometres travelled (litres/100km). Many drivers calculate it by monitoring the distance driven between fill-ups. Driving on the road is not a good way to determine how fuel economy is affected by petrol composition because it is difficult to control the many other factors that are involved. A more accurate determination is possible under controlled laboratory conditions. Vehicles are mounted on a chassis dynamometer in a temperature-controlled space and driven through a specified operating cycle. The weight or volume of the petrol consumed during the cycle may be measured or it may be calculated from the weight fraction of carbon compounds in the vehicle's exhaust.

This diagram shows how the average fuel economies of two fleets of vehicles are proportional to the heating values of the petrols tested. This is the relationship predicted by combustion theory. The newer fleet was composed of 1989 model-year cars; the older fleet of 1984-1985 model-year cars. The test involved two different sets of petrols (Matrix A and Matrix B) which varied in aromatics content, olefins content, oxygen content, oxygenate type, and several other properties. This result shows that heating values can be used as surrogates for actual fuel economy measurements when considering the effect of petrol composition on fuel economy.

Conventional fuels have always varied in heating value. One cause is the formulation differences among batches and among refiners. A survey of 1990-1991 conventional petrols found that the heating value of summer petrols varied over an eight percent range. The heating value also varies by grade and by season. On average, the heating value of premium-grade petrol is about 0.7% higher than regular-grade because premium-grade, in general, contains more aromatic hydrocarbons - the class of hydrocarbons with the highest densities. The heating value of winter petrol is about 1.5% lower than summer petrol because winter petrol contains more volatile, less dense hydrocarbons.

Factors Affecting Fuel Economy

Fuel economy is affected by a vehicle's size, weight, aerodynamics, fuel delivery system, engine type and transmission type. These values remain constant for a specific vehicle.

There also are many variable factors. As mentioned, the heating value of the petrol is one. In addition, fuel economy is affected by weather conditions, air conditioner use, road conditions, the route driven, driving speed and driving style. And it is affected by the mechanical condition of the car - engine tune, wheel alignment and tyre pressure. Some of these non-petrol factors have the potential to cause substantial changes in fuel economy. The table below shows a list of average and maximum effects published by the United States Environmental Protection Agency (EPA).

Winter-related factors can combine to lower fuel economy 20% compared to the summer. Rain or snow on the road offers more resistance to the tires. It may also require the driver to slow for safety to a less fuel-efficient speed. In cold weather, a richer air-fuel mixture is required to start and warmup the engine. And much of the warmup is done at idle because of the need to defog or defrost the windows. Also, in many vehicles, the air conditioner is operated to assist defogging. More energy is required to overcome the resistance created by the higher viscosities of cold lubricants - engine oil, transmission fluid, and differential lubricant.

Short trips are worse for fuel economy than long trips because a cold engine uses more fuel than a warm engine and because of the energy required to overcome the resistance of cold lubricants. Traffic jams and bumper-to-bumper driving also extract a heavy toll on fuel economy.

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