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Sector: CNG

Skoda Praktik diesel-gas - dual practicality

Skoda Praktik diesel-gas - dual practicality ©

Recently we took the opportunity to test a diesel-powered Skoda Praktik equipped with a DegaMix II gas injection system by Elpigaz. The product is currently undergoing comprehensive tests in a number of research vehicles. The Skoda van has been converted to run partially on CNG (compressed natural gas), but an LPG (liquefied petroleum gas) version of the 2nd generation DegaMix will also be available. The system has been designed for common-rail oil-burners.
Skoda Praktik diesel-gas© gazeo.comSkoda Praktik TDI equipped with a DegaMix II CNG system by Elpigaz

Having gathered certain data, experience and feedback from the first generation of the DegaMix diesel-gas blending system, Elpigaz now introduces its second iteration. The new one is much more precise, which – in addition to the pressure sensor signal emulation option – makes it possible to remarkably boost gaseous fuel ratio in the overall amount of fuel delivered to combustion chambers. Additionally, by using CNG instead of LPG, gaseous fuel dose may be raised up to 40 percent. With this kind of displacement ratio, converting a diesel to partially run on gas may appeal financially to a broader group of fleet operators than to-date diesel-gas blending systems, generally best-suited for heavy-duty trucks. The DegaMix II, on the other hand, is supposed to appeal to drivers of light-duty delivery vans and passenger cars. The Praktik has been converted as a demonstration vehicle to convince exactly them.

Elpigaz DegaMix II ECU© gazeo.comThe DegaMix II ECU

The car
The Roomster's utility counterpart has been equipped with a 66 kW/90 PS 1,6 TDI motor, offering its peak power at 4200 RPM and its peak torque – 230 Nm – as early as at 1500 RPM.

The gas system
The Praktik's gas fuel system has been tuned to boost fuel economy, i. e. to displace as much diesel as possible in favour of cheaper and cleaner compressed natural gas. The system's ECU complies with the 67 R01 and the 110 EEC UN regulations, which means it can be used for controlling both LPG and CNG gas injection systems. In this case a CNG one is used.

The reducer
CNG pressure reduction is the job of the AT12 reducer by Tomasetto. It is a two-stage reducer feeding gas to injectors at constant pressure, regardless of the engine's temporary fuel supply needs. The

DegaMix II system components© gazeo.comI-plus injectors, gas pressure sensor and gas filter


pressure can be regulated within the 1-2,5 bar range depending on pressure in the intake manifold. The reducer is efficient enough to supply engines with up to 250 PS of power.

The I-plus injectors

Once the pressure of CNG is reduced, the fuel is filtered and then supplied to two I-plus injectors, injecting natural gas into the intake manifold via diagonally-mounted nozzles. As for the injectors themselves, their construction is based on light plates instead of common pistons. Thanks to this they need very little time to open. They are also supposed to be very little prone to mechanical wear and their longevity is estimated by the manufacturer at 90 thousand km (until first maintenance).

The multivalve of the CNG tank© gazeo.comThe CNG tank's multivalve, regulating the vessel's proper and safe operation

The pressure tank
CNG is stored at 20 MPa in a single steel tank mounted longitudinally in the cargo space, behind the driver's seat. The tank's gross capacity is 50 l, which means it can hold approximately 12 cubic metres of compressed natural gas. It has been fitted with a multivalve comprising the following elements:

- a safety valve rated at 33 MPa pressure;
- a fire valve, allowing gas escape when the tank's temperature reaches 110° C;
- an excessive flow cut-off valve;
- a manually-operated maintenance valve;
- an electromagnetically-operated maintenance valve.

  • How the DegaMix II works
    Basically, the DegaMix II is a variant of AEB's MP48 DF (Dual Fuel) ECU made specifically for Elpigaz. Apart from controlling gas injectors, it can process the diesel pressure sensor signal and forward it to the engine's original ECU. This way the OEM ECU cuts the diesel dose and makes it possible to inject CNG in its place. Thanks to this operating strategy, relatively high replacement ratios (40 percent and more) can be achieved. However, as compared to its predecessor, the DegaMix II has one flaw – it can only be used to convert common rail-type diesel engines. But since most oil burners use common rail fuel supply strategy these days, this flaw will soon self-eliminate.

. Calibration. Once the gas system was activated, the engine's parameters were virtually the same as on diesel alone, which was confirmed by our dyno tests. A diesel-gas blending system can be tuned to give a power and torque boost, but this is not the case here – the one in the Praktik is there to cut down the fuel costs as much as it can by replacing as much diesel with CNG as technically possible. In fact, boosting power and torque could mean boosting economy, too, but the…

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Piotr Złoty
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