CNG-NGV is suitable for most Petrol (Gasoline) Engine
CNG-NGV is suitable for most Petrol (Gasoline) Engine
Written and Presented by Dr. Xander Thong (MVLK Certified Gas Installer, Malaysia)
At NGV Community.com, we are frequently asked many questions pertaining to CNG-NGV conversion suitability by vehicle Make and Model. The questions comes in various forms, but they can be summarised to "What engine types are most suitable for CNG-NGV conversion into Bi-Fuel?".
CNG-NGV Conversion determination is based on engine type, fuel management system and bi-fuel assimilations. Within this topic, we will attempt to maintain a simplified explanation on technical issues, and address areas that is most important for your understanding about CNG-NGV Bi-Fuel conversion and modifications. This is essential as you are required to possess a basic fundamental understanding of how an engine type, spark ignition system and CNG properties together plays a major part in having a bi-fuel vehicle.
In these topics, we will explain the basic outline of:
(a) Internal Combustion Engine : Otto-Cycle Engine (most common engine in everyday vehicles)
(b) CNG Properties for Engine-Type Conversion and Modification
(c) Auto Spark Ignition System : Timing Advance for Spark Ignition
ENGINE 101 - SUITABLE ENGINE-TYPE FOR CNG-NGV CONVERSION
An internal combustion engine is a type of machine that obtains its kinetic (movement) energy directly from the burning (or combusting) of chemical (such as petrol\gasoline, diesel, CNG, LPG, Hydrogen) energy in a Combustion Chamber. There are 4 major engine types that are in use today:
(a) Otto-Cycle or 4-Stroke Engine (common to passenger vehicles)
(b) Diesel-Cycle or Diesel Engine (common to most medium and heavy-duty commercial vehicles)
(c) Wankel-Cycle or Rotary Engine (common to some Mazda models)
(d) Gas-Turbine Engine (common to aircarft engines)
OTTO-CYCLE ENGINE - YOUR CAR'S ENGINE
The best engine-types suitable for CNG-NGV conversion is Otto-Cycle engine. This engine type is dominant in most (if not all) passenger vehicles in use today.
The Otto-Cycle Engine was invented by Nikolaus Otto in 1876, and it is also commonly known as a 4-Stroke engine. The 4-Stroke cycle is more fuel-efficient and clean burning than the 2-stroke cycle engine. However so, 4-stroke cycle engine requires considerably more moving parts and better manufacturing expertise. This engine is more characterised by a four-strokes, or straight movement in a single direction, of a piston in a cylinder:
Stroke (1) - Intake Stroke;
Stroke (2) Compression Stroke;
Stroke (3) Ignition (power-expansion) Stroke; and
Stroke (4) Exhaust Stroke.
The cycle begins at TOP DEAD CENTER (TDC), when the piston is at its topmost point. On the first downward stroke (1) or Intake Stroke of the piston, a mixture of air and fuel (petrol or CNG) is drawn into the cylinder through the intake valve or valves. The intake valves then closes, and the following upward stroke (2) or Compression Stroke compresses the air-fuel mixture.
TDC Stroke 1 Stroke 2
Next, the air-fuel mixture is ignited with a spark plug for petrol/gasoline or CNG engine at approximately the top of the Compression Stroke or Stroke (2). The result is an explosion of air-fuel mixture and expansion of burning gases that force the piston downward - which is known as the Stroke 3 or Ignition Power Stroke. The Stroke (4) or Exhaust Stroke evacuates the spent or residue gases out and away from the cylinder through the opening of the exhaust valve or valves.
Stroke 3 (Ignition)Stroke 3 Stroke 4
A Brief Operation of the Otto-Cycle Engines:
The ordinary Otto-cycle engine is a four-stroke engine; that is, to complete a cycle, it's pistons make four strokes, two toward the head (closed head) of the cylinder and two away from the head. During the first stroke of the cycle, the piston moves away from the cylinder head while simultaneously the intake valve is opened. The motion of the piston during this stroke sucks a quantity of fuel and air mixture into the combustion chamber. During the next stroke, the piston moves toward the cylinder head and compresses the fuel mixture in the combustion chamber. At the moment when the piston reaches the end of this stroke and the volume of the combustion chamber is at a minimum, the fuel mixture is ignited by the spark plug and burns, expanding and exerting pressure on the piston, which is then driven away from the cylinder head in the third stroke. During the final stroke, the exhaust valve is opened and the piston moves toward the cylinder head, driving the exhaust gases out of the combustion chamber and leaving the cylinder ready to repeat the cycle.
The efficiency of a modern Otto-cycle engine is limited by a number of factors, including losses by cooling and by friction. In general, the efficiency of such engines is determined by the compression ratio of the engine. The compression ratio (the ratio between the maximum and minimum volumes of the combustion chamber) is usually about 8 to 1 or 10 to 1 in most modern Otto-cycle engines. Higher compression ratios, up to about 15 to 1, with a resulting increase in efficiency, are possible with the use of high-octane anti-knocking fuels. The efficiencies of good modern Otto-cycle engines range between 20 and 25 percent—in other words, only this percentage of the heat energy of the fuel is transformed into mechanical energy.
CNG Properties for Engine-Type Conversion and Modification
In CNG-NGV conversion on petrol (gasoline) engines, you must at least be able to understand the following terms and its impact on CNG-NGV operation:
(a) Compression Ratio (specifies how much the fuel is compressed when the engine's piston is at its highest point or TDC).
(b) Auto Ignition Temperature (specifies the temperature at which a material [i.e., solid, liquid, or gas] will self-ignite and sustain combustion in air without an external spark or flame.)
(c) AutoIgnition Timing (specifies a system in an internal combustion engine that initiates the chemical reaction between fuel and air in the cylinder charge by producing a spark).
(d) Octane Number (specifies a value used to indicate the resistance of a fuel to engine knocking or pinging. Octane numbers are based on a scale on which isooctane is 100 (minimal knocking ) and heptane is 0 (bad knocking).
(e) Knocking (also called pinking or pinging)—technically detonation—in internal combustion engines occurs when fuel/air mixture in the cylinder has been ignited by the spark plug and the smooth burning is interrupted by the unburned mixture in the combustion chamber exploding before the flame front can reach it. Combusting stops suddenly, because of the explosion, before the optimum moment of the four-stroke cycle. The resulting shockwave reverberates in the combustion chamber and pressures increase catastrophically, creating a characteristic metallic "pinging" sound.
As per your previous reading on CNG-NGV, it is very likely that you have come across a Fuel Properties Table as shown above. For non-automotive readers, the numbers presented are great, but what does it actually mean. Here is the part where NGV Community.com demystifies the Dark Art of CNG-NGV, starting with Compression Ratio, Auto Ignition Temperature, Octane Rating and Auto Ignition Timing.
IMPACT (A) - CNG-NGV CONVERSION AND ENGINE PARAMETER READJUSTMENTS
The Compression Stroke or Stroke (2) is the upwards movement of the piston in the cylinder with the valves closed (air-tight). This upwards motion compresses the fuel air mixture inside the combustion chamber raising the pressure. The difference between the initial volume of the cylinder and the final volume at the top of the compression stroke is known as the compression ratio. The compression ratio is particularly important in achieving the desired Auto Ignition Temperature. Higher pressure level increases Auto Igniton Temperature as air-fuel molecules are compacted nearer together.
As with petrol (gasoline), compression ratio of 10 : 1 is sufficient to obtain an Auto Ignition Temperature around 200 oC. If the compression ratio is raised to 12.5 : 1 in a air-petrol mixture, the temperature will be increased significantly, which can lead to an unnecessary pre-detonation in the cylinder chamber before TDC as the Octane rating is low (at 89, some 97). Knocking will appear as the pre-detonation pushes the piston downwards while it is moving upwards to reach TDC. As a result, a counter-force shockwave reverberates in the combustion chamber and pressures increase catastrophically, creating a characteristic metallic "pinging" sound.
As with CNG-NGV, it is necessary to increase the compression ratio to obtain the desired Auto Ignition Temperature of 540 oC. At this temperature, air-fuel (CNG) mixture will combust most efficient, given that other parameter are met appropriately. In a simplier terms, Compression Ratio on all modified CNG-NGV vehicles must be increased to the range of 10.5 to 12.5:1.
Rule of Thumb: CNG-NGV Compression Ratio
Increase Compression ratio = Increase Chamber Pressure = Increase Auto Ignition Temperature = Increase Efficiency
(in achieving CNG-NGV combustion efficiency)
Honda Civic GX (Monofuel) CNG + Petrol (Gasoline) Compression Ratio, Download Here
Higher compression ratio will indeed increase the Auto Ignition Temperature for CNG in obtaining a desireable 540 oC per cylinder chamber. However so, CNG will not be easily pre-detonated by extreme heat at 540 oC as petrol (gasoline) would. The octane rating of 120 makes CNG a much higher resistant fuel that deters pre-detonation by heat. It requires a spark plug at Stroke 3 (Power-Ignition) to complete the combustion process. The timing on the spark ignition (or Ignition Timing) is also crucial as CNG has a strong resistance to pre-detonation (and to some degree of pre-ignition).
Vehicle Ignition System
Written and Presented by Dr. Xander Thong, MLVK Certified Gas Installer (Malaysia)