Thursday, December 4, 2014

GDI – Gasoline Direct Injection

GDI is the direct injection of gasoline (fuel) into the combustion chamber typically using a High Pressure Common Rail (HPCR) system.

HPCR uses a high pressure pump, typically above 15,000 psi (1034 bar) and potentially as high as 55,000 psi (3800 bar) to supply fuel to piezoelectric injectors. These injectors use stacks of ceramic crystal cells capable of producing up to nine (9) injections per combustion cycle. These cells expand slightly with an electrical charge and contract when that charge is removed. By stacking many cells together the small expansions and contractions are combined to create a movement sufficient to open and close an Injector Pintle in very consistent, very fast cycles.

Using one or more computer modules to control the fuel system (and several other operating parameters) allows the engine to create an Ultra lean burn or Stratified Charge condition where the normal stoichiometric ratio of 14.7:1 can be extended as high as 65:1 for short periods under light load or deceleration, Stoichiometric Condition during moderate load, and Power Condition where slightly richer than stoichiometric fuel ratios exist.

                This creates conditions allowing for much greater power output per given displacement (*higher power density), which can be translated into dramatically improved fuel economy, while significantly lowering emissions. This allows for example a six cylinder engine to produce the same (or more) horsepower and torque than a much larger 8 cylinder engine, but with a smaller size and less weight.

                All of this high technology equipment brings with it some challenges and problems. The move to GDI has caused some unexpected problems, the biggest one being “Intake Valve Deposits”.

                Intake Valve Deposits are caused by a combination of problems found in all gasoline engines’ that did not however become apparent until GDI was introduced.

                In all internal combustion engines, a small amount of fuel bypasses the piston rings, washing down the cylinder walls and into the crankcase. In the days before we started with Ethanol blended gasoline, that small amount of gasoline would vaporize and be sucked up by PCV (Positive Crankcase Ventilation) system. That gasoline mixed with some vaporized motor oil would be burned in the engine with relatively little trouble.

However with the addition of Ethyl Alcohol (Ethanol) to gasoline has made the situation worse in several ways. First, Ethanol mixed with motor oil liberates (breaks down) some of the important additives found in motor oil. Components such as Phosphorous, ZDP or ZDDP (Zinc), and Sulfonated Ash separate from the oil and are then easily vaporized. The PCV system is powered by vacuum from the intake system and all of that vaporized material flows into the intake and flows over the Intake Valves. When that material hits the very hot intake valves, it condenses and then sticks forming rock hard carbon deposits on the valve stems and in and around the port area.

                In earlier engines with Port Fuel Injection, Sequential Fuel Injection, and Carburetors, fuel was mixed with air and traveled through some or all of the intake system which allowed that fuel to continuously wash over and clean the intake ports and intake valves. This did an excellent job of keeping these areas and components clean.

                Now in engines using GDI the fuel is directly injected into the combustion chamber bypassing the intake, intake port, and intake valves so there is nothing to wash or help keep the valves clean.

                Next, Ethanol is Hygroscopic, meaning that it actually picks up moisture from the atmosphere and from condensation inside the engine and holds it in suspension. As the amount of water in the oil increases, you will reach a point where something called Phase Separation takes place, where the Ethanol and Water blend together and separate from the Motor Oil and any residual gasoline forming a distinct layer at the bottom of the crankcase.

This Ethanol/Water layer is a highly corrosive emulsion that will rapidly corrode the internal engine components and can have several other insidious effects. The layer is at the very bottom of the crankcase (underneath the motor oil) and this is where the oil pump pickup tube is positioned. When the engine is started, the oil pump immediately picks up this bad layer and pumps it throughout the engine. This can cause increased wear and will actually strip lubrication from all the engines moving parts.

Worse yet, if you live in area where the temperatures get below freezing, the water can freeze, actually blocking the pickup tube and starving the engine from all lubrication. Either of these situations can cause catastrophic engine failure.

There are several proactive things that can reduce or eliminate many of these concerns. The first involves a thorough cleaning of the inside of the engine.