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How much gasoline does an engine need?

Once we understand the operation of the entire mechanical assembly and how the air moves inside the engine, it is time to change stage. From now on we will add fuel to this mixture to power the engine. We start an extensive, and often complex series. But that I will try to treat in the best possible way.

Today’s talk lays the foundation on the whole subject. It will serve as an introduction to ones and review for others. But let’s follow that we have a lot to walk.

Let’s start by talking about the size of things, the measurement nomenclature so that there are no doubts or errors when reading or classifying a measurement. For this we have to know the scales, that is, the multiples or dividends of a standard measure. For example, when you see the announcement of a phone with 32 Gig memory (imagine the announcer shouting), run away. Giga is just one of the scales used to define the size of a measure. Which in this case is the byte. That is, 32 gigabytes and not gigas. The Giga scale means that we have nine representative zeros. If we want to put exponentially we have 10⁹. So the 32 Gb means thirty-two billion bytes available for you to store data. But the scales also manifest themselves in the division of measure. For example, a centimeter is the hundredth part of a meter, a millimeter is the thousandth part.

Now that we understand the scales let’s talk about the measures themselves. There is an international standard for each. It is the International System of Measures, or simply S.I. But there is also the imperial system, or English. Used mainly in the lands of Uncle San and the Queen, the units have relations with measures of the medieval period and even before that. For example, the foot is the standard length measurement, which comes from ancient Greece, equaling 0.3038 meter. In the world of gears and oil we find the values ​​basically in these two measurement standards.

Do you know the “clock” we sometimes find in turbo cars, which indicate the vacuum phase and the supercharged phase (pressure) of the engine? That’s a manovacuometer. There are basically two ways of measuring pressure. The absolute, where we consider as zero the total absence of pressure. This condition is called absolute zero and all values ​​on this scale are positive.

In order to differentiate the values ​​measured in absolute pressure, each measuring unit receives a subscript “a” (PSIₐ). The second form of measurement is the gauge, where the atmospheric pressure value is considered as zero. Any value measured below this is interpreted as vacuum, depression or negative pressure. When we read values ​​of gauge pressure in most cases there is no subscription, but we can find in older articles a “g” subscribed. See the example below for the best understanding.

14.69 PSIₐ = 0 PSI

8.8 PSIₐ = -5.9 PSI

The pressure gives us reference to the density when we analyze a gas. Under the prism of the engines the pressure associated with the flow forms the basis for the engine’s fuel adjustment.

Let’s talk about flow. By definition it is the amount of fluid that travels through the section of a duct for a certain time. Slightly vague, is not it? Let’s simplify. Imagine a bottle with a known volume, say 10 liters. Now watch a faucet pouring water into the carafe. This one gets full in a minute. Then the water flow through the faucet is ten liters per minute. I think that makes it easier to understand.

This condition that we have just described is the volumetric flow, that is, the volume of fluid measured as a function of time. But beyond it we have the mass flow, where we have the measured mass as a function of time. In the world of motors the flow is a key factor to tune the whole. And for us to be as accurate as possible, it is essential to use the mass flow for fuel adjustment.

Temperature is the measure of the amount of heat emitted or absorbed by a body. For our needs the temperature brings references in the correction phase to the amount of fuel initially added to the air stream. Values ​​such as the coolant temperature, air inlet temperature and exhaust gas temperature are essential to keep the mixture closer to the ideal values.

It is the condition that describes the ideal proportion between the chemical elements. In the specific case of this chat it designates the balance between the air and the fuel to carry out the complete combustion. It is represented by the Greek letter lambda (λ). Most of us know that the ratio of air / fuel to gasoline is 14.7: 1. But how the hell was this reason found? Well, for that we need to reminisce about the chemistry. The commonly used fuel molecules found in our tanks are composed of combinations of carbon and hydrogen. Gasoline, diesel and CNG are hydrocarbons, but there are also oxygenated compounds such as Ethanol, Methanol and Nitromethane

We will use Ethanol as a reference for our initial calculations, because unlike gasoline only one molecule makes up this alcohol. Its formula is C₂H₅OH, and its molecular weight is 46.068 daltons. Combustion is a rapid oxidation process, so we need oxygen to perform it. The result of the complete combustion is carbon dioxide (CO₂) and water (H₂O). So we need to balance the molecules for the right result. So:

With this we have completed the necessary fundamentals for the understanding of the fuel supply system and its adjustments. In the next discussion we will deal with the carburetors and mechanical injection. History, construction, secrets and adjustments. See you next time!


  1. I got what you intend, appreciate it for posting.Woh I am thankful to find this website through google.

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