Fuel Anatomy

Introduction

Nowadays, what most people think about petrol? Either by remembering to fill up when empty or checking on the price before refilling. Since the petrol is always functioning and performing well, most people forget what a sophisticated petrol really is.

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

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

Volatility

Driveability describes 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 is expecting.

Driveability problems include:
- Hard to start,
- Backfiring,
- Rough idle,
- Poor throttle response,
- Stalling.

The key petrol characteristic for a good driveability and volatiity is the petrol’s tendency to vaporize. Volatility is very important because liquids and solids don’t burn, only vapors burn. In cold weather, petrol is blended to vaporize easily. This allows an engine to start quickly and run smoothly until it is warm. During warm weather, petrol is blended to vaporize less easily in the vehicle to prevent vapor lock or other hot-fuel handling problems and it also minimize the evaporation, which contributes to air pollution.

Anti-Knock Performance

Knock-free engine performance is as important as good driveability:
- Octane number is a measure of a petrol’s anti-knock performance. It’s 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.

Knocking is the sound of abnormal combustion

- Normal combustion in a spark-ignition internal combustion engine is initiated by a spark.
- The flame front fans out from the spark plug and travels across the combustion chamber rapidly and smoothly until almost all the fuel is consumed.
- When combustion is abnormal, the last part of the unburned mixture ignites spontaneously and burns very rapidly, causing the pressure in the cylinder to rise rapidly. This creates the characteristic knocking sound.
- Occassional light knocking doesn’t hurt the engine.
- Heavy or prolonged knocking can results in:
– Objectionable noise
– Loss of power
– Overheating of engine parts
– Knocking-induced surface ignition
– Engine damage
- Knock occurs because the petrol’s octane number is below the anti-knock requirement of the engine at that moment. Generally, the situation involves high-load conditions, like hard acceleration or climbing a grade.

Vehicles are designed and calibrated for a certain octane value

When a customer uses petrol with an octane level lower than that required, it may results in knocking. Modern engine equipped with knock sensors and can handle lower octane levels by retarding the spark timing. However, the fuel consumption, driveability and power may suffer. Using petrol with an octane rating higher than the recommended ones, doesn’t means it may improve the vehicle’s performance.

Fuel quality is becoming increasingly important. Without the right fuel, the engine won’t perform as well as it was designed to perform

Fuel quality can be an enabler for the introduction of new technologies:
- Lower sulfur levels can enable new after-treatment technologies (catalysts)
- Higher octane fuels can enable higher compression more fuel efficient vehicles
- High detergency fuels can enable accurate and precise fuel delivery to the combustion process over the life of the vehicle and it also help in assist the autos meet the performance, fuel economy and emissions goals.

From http://triplepower.net/?p=197