Common Rail Direct injection (CRDi Engines)

CRDI: Widely know as CRDi, CRDe, DICOR and Tdi and so on by various manufacturers, it has been marked as the technology for this week. The reason, as we can see all around us the fact that the modern diesel is no longer a smoke breathing rattling workhorse, but is the biggest buzzword in the Indian car market. This technology, coupled with rising fuel prices of course has made a diesel car both economical and enjoyable.
 

First conceived by Marelli in 1987 but it wasn't until Fiat acquired the technology that it got tested on one of their cars in 1992. The process was later sent to Bosch group for implementation in the commercial market. The most important contributor to its effectiveness remains with the common rail. Which basically means same fuel line. That is, when fuel gets injected into say cylinder 1; there is virtually no pressure loss at the rest of the rail. This in turn means that there is always adequate pressure available for the injection. Common rail technology makes use of 2 pumps in order to bring the fuel up to high pressures of up to 1350 bar. During the first stage, an electronic pump draws required amount of fuel from the fuel tank; this low-pressure pump is governed by the engine management system. The speed of the pump is determined by driver inputs and other information obtained from sensors. This has allowed Common rail systems to reduced emissions due to absence of unburnt fuel. The second stage of pumping is done with the help of a mechanical pump that is coupled with the crankshaft and geared in order that it may rotate at half engine speed. The fuel now goes to an accumulating duct (rail), where these pressures maybe maintained. This tank allows for the maintaining of this constant pressure even during the injection. The injection maybe carried out using electromagnetic valves, which govern the exact amount of fuel for injection. Leaks occurring at the pump, leaks for opening the valves etc are returned back to the fuel tank. Which in turn results in zero wastage of fuel.

When common rail technology is integrated with turbo chargers or superchargers the power delivered by a diesel engine may well exceed that attained by a similar sized petrol engine. Today's manufacturers are embracing this technology due to all the advantages it holds. At the rate with which this technology is catching up, Diesel definitely holds the key as far as development in efficiency is .

Principle: 

Solenoid or piezoelectric valves make possible fine electronic control over the fuel injection time and quantity, and the higher pressure that the common rail technology makes available provides better fuel atomisation. In order to lower engine noise the engine's electronic control unit can inject a small amount of diesel just before the main injection event ("pilot" injection), thus reducing its explosiveness and vibration, as well as optimising injection timing and quantity for variations in fuel quality, cold starting, and so on. Some advanced common rail fuel systems perform as many as five injections per stroke.

Common rail engines require no heating up time and produce lower engine noise and emissions than older systems.

Diesel engines have historically used various forms of fuel injection. Two common types include the unit injection system and the distributor/inline pump systems. While these older systems provided accurate fuel quantity and injection timing control they were limited by several factors:

• They were cam driven and injection pressure was proportional to engine speed. This typically meant that the highest injection pressure could only be achieved at the highest engine speed and the maximum achievable injection pressure decreased as engine speed decreased. This relationship is true with all pumps, even those used on common rail systems; with the unit or distributor systems, however, the injection pressure is tied to the instantaneous pressure of a single pumping event with no accumulator and thus the relationship is more prominent and troublesome.
• They were limited on the number of and timing of injection events that could be commanded during a single combustion event. While multiple injection events is possible with these older systems, it is much more difficult and costly to achieve.
• For the typical distributor/inline system the start of injection occurred at a pre-determined pressure (often referred to as: pop pressure) and ended at a pre-determined pressure. This characteristic results from "dummy" injectors in the cylinder head which opened and closed at pressures determined by the spring preload applied to the plunger in the injector. Once the pressure in the injector reached a pre-determined level, the plunger would lift and injection would start.

In common rail systems a high pressure pump stores a reservoir of fuel at high pressure — up to and above 2,000 bars (29,000 psi). The term "common rail" refers to the fact that all of the fuel injectors are supplied by a common fuel rail which is nothing more than a pressure accumulator where the fuel is stored at high pressure. This accumulator supplies multiple fuel injectors with high pressure fuel. This simplifies the purpose of the high pressure pump in that it only has to maintain a commanded pressure at a target (either mechanically or electronically controlled). The fuel injectors are typically ECU-controlled. When the fuel injectors are electrically activated a hydraulic valve (consisting of a nozzle and plunger) is mechanically or hydraulically opened and fuel is sprayed into the cylinders at the desired pressure. Since the fuel pressure energy is stored remotely and the injectors are electrically actuated the injection pressure at the start and end of injection is very near the pressure in the accumulator (rail), thus producing a square injection rate. If the accumulator, pump, and plumbing are sized properly, the injection pressure and rate will be the same for each of the multiple injection events.