Editing Common rail (section)

==Principles==
[[File:Common Rail Scheme.svg|350px|right|thumbnail|Diagram of the common rail system]]
Solenoid or [[piezoelectric]] valves make possible fine [[electronic control unit|electronic control]] over the fuel-injection time and quantity, and the higher pressure that the common rail technology makes available provides better fuel [[aerosol|atomisation]]. To lower engine [[noise (environmental)|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.<ref>(multistroke injection) See BMW 2009 Brochure for 3 series</ref>

Common rail engines require a very short to no heating-up time, depending on the ambient temperature, and produce lower engine noise and emissions than older systems.<ref>{{cite web |url=http://www.carservicesalisbury.com/go/common-rail-diesel-service%7D |website=www.carservicesalisbury.com |access-date=15 January 2022 |title=Archived copy |archive-date=14 May 2018 |archive-url=https://web.archive.org/web/20180514134251/http://www.carservicesalisbury.com/go/common-rail-diesel-service%7D |url-status=dead }}</ref>

Diesel engines have historically used various forms of fuel injection. Two common types include the [[Unit injector|unit-injection]] system and the [[Injection pump|distributor/inline-pump systems]]. While these older systems provide accurate fuel quantity and injection timing control, they are limited by several factors:

*They are cam driven, and injection pressure is proportional to engine speed. This typically means that the highest injection pressure can only be achieved at the highest engine speed and the maximum achievable injection pressure decreases as engine speed decreases. This relationship is true with all pumps, even those used on common rail systems. With unit or distributor systems, the injection pressure is tied to the instantaneous pressure of a single pumping event with no accumulator, thus the relationship is more prominent and troublesome.
*They are limited in the number and timing of injection events that can be commanded during a single combustion event. While multiple injection events are possible with these older systems, it is much more difficult and costly to achieve.
*For the typical distributor/inline system, the start of injection occurs at a predetermined pressure (often referred to as pop pressure) and ends at a predetermined pressure. This characteristic results from "dumb" injectors in the cylinder head which open and close at pressures determined by the spring preload applied to the plunger in the injector. Once the pressure in the injector reaches a predetermined level, the plunger lifts and injection starts.

In common rail systems, a high-pressure pump stores a reservoir of fuel at high pressure — up to and above {{convert|2000|bar|MPa psi}}. The term "common rail" refers to the fact that all of the [[fuel injector]]s 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 needs to maintain a target pressure (either mechanically or electronically controlled). The fuel injectors are typically controlled by the [[engine control unit]] (ECU). 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.

Third-generation{{vague|date=October 2020}} common rail diesels now feature [[piezoelectric]] injectors for increased precision, with fuel pressures up to {{convert|2500|bar|MPa psi|abbr=on}}.<ref>{{Cite news|url=http://www.globaldenso.com/en/newsreleases/130626-01.html|title=DENSO Develops a New Diesel Common Rail System With the World's Highest Injection Pressure{{!}} News {{!}} DENSO Global Website|work=DENSO Global Website|date=2013-06-26|language=en-US|url-status=live|archive-url=https://web.archive.org/web/20171013221631/http://www.globaldenso.com/en/newsreleases/130626-01.html|archive-date=2017-10-13}}</ref>