A fuel with many advantages

There is great po­ten­tial for Com­pres­sed Natural Gas (CNG) due to it hav­ing very low emis­sions. Com­pared to gaso­line, CNG com­bus­tion pro­duces about 25% less car­bon di­ox­ide (CO₂). Fur­ther­more, CNG has a great po­ten­tial for re­duc­ing un­treated emis­sions. The ex­haust gas is odor­less and con­tains no par­ti­cles.

CNG also has an ad­van­tage when be­ing pre­pared as a fuel: it needs no ad­di­tives and its pro­duc­tion does without com­pli­cated re­fin­ing pro­cesses. Another ad­van­tage is that CNG vehicles place no load on the world's scanty pe­tro­leum re­sources. Methane, CNG's ma­jor compo­nent, can also be pro­duced from or­ganic sub­stances. This closes the CO₂ cy­cle and long-term avail­ability is in­creased even fur­ther.

CNG vehicles have proven them­selves for years now. Since CNG fill­ing sta­tions are still few and far be­tween, the vehicles are mostly equipped with bi-fuel sys­tems, and the en­gine can run on ei­ther natural gas or gas­oline.

CNG has a very high knock resis­tance (130 ROZ as op­posed to be­tween 91 and 100 ROZ for gas­oline). This re­presents fur­ther potential for op­timiza­tion of the CNG en­gine. This is ide­ally suited for super­charging, al­low­ing down­sizing concepts to be ap­plied with the ac­compa­ny­ing im­prove­ments in effi­ciency.

CNG engine-management system

In or­der to be suc­cess­ful on the market, in ad­di­tion to their ad­van­tages re­gard­ing environ­mental compati­bil­ity CNG vehicles must also fea­ture good dy­namic re­sponse, high driv­ing com­fort, and suitabil­ity for every­day use. Bosch sup­ports these re­quire­ments by de­vel­op­ing an engine-man­agement sys­tem for CNG vehicles. This com­prises the engine-man­agement ECU for the bi-fuel sys­tems (CNG and gas­oline), as well as com­po­nents like pressure regu­lator module, tank valve, low- and high pressure sensors. During de­vel­opment, the em­pha­sis was on two main points. Firstly the switch­ing be­tween CNG and gas­oline op­era­tion had to take place with­out any ef­fects on torque, and sec­ondly, a simple OBD concept was needed. The compo­nents were manu­factured and tested ac­cording to Bosch quality standards, and certi­fied in line with the valid ECE-R110 Standard for CNG vehicles.

The bi-fuel NG-Motronic ECU

This ECU is bas­ed on the Mo­tronic ver­sion for gas­oline in­jec­tion. It con­trols the CNG in­jec­tors via separate dri­ver sta­ges. The torque-guided con­trol permits the simple in­te­gra­tion of the func­tions which are spe­cific for CNG opera­tion. A num­ber of ad­van­tages re­sult from inte­gra­ting both fuel sys­tems in a bi-fuel ECU. For in­stance, low­er wir­ing costs, and cost sav­ings due to the sec­ond ECU be­com­ing un­nec­es­sary. In addi­tion, the sys­tem as a whole has con­sid­er­able op­timiza­tion po­ten­tial.

A fur­ther ad­van­tage in­her­ent in the sin­gle-ECU con­cept is the fact that at every op­erat­ing point a co­ordi­nated change can be made be­tween gas­oline and CNG op­era­tion without any jump in torque.

The NGI2 natural-gas injector

The first Bosch natural-gas in­jec­tion valve came onto the mar­ket years ago. The new NGI2 in­jec­tor was spe­cifi­cally de­vel­op­ed for the de­mands of auto­mo­tive CNG in­jec­tion. It fea­tures a se­ries of techni­cal in­nova­tions and serves as a bench­mark for gas me­ter­ing. The compo­nents through which gas flows are de­signed to cope with the high gas vol­umes and the high through­flow veloc­ity. Pres­sure los­ses be­fore the throt­tling point and op­era­ting noise are kept to a minimum by a spec­ial flow guide.

The solenoid can be trig­gered by a standard driver stage, and its sur­face has been spec­ially coated to pre­vent wear when used with oil-free gas.

With its spe­cial de­sign, the NGI2 is far super­ior to all pre­vi­ously known gas-in­jec­tion valves, and can be eas­ily inte­grated in exis­ting intake-mani­fold ge­ometries.