GETTING THE BEST OUT OF NATURAL OAS I Natural gas
Page 19
If you've noticed an error in this article please click here to report it so we can fix it.
has a number of intrinsic advantages as a fuel, but they can be negated if the engine is not optimised to use it, Bi-fuelled 51 engines can run on petrol or natural gas, but as the two fuels have different characteristics the designers have to favour one fuel or the other. Dual-fuelled engines are based on diesels and are designed to run on a mixture of diesel and natural gas.
The engine usually starts on diesel, and the proportion of CNG increases with engine speed and load. Again,
Air excess ratio ( lambda)
the design is a compromise. If you want to reduce emissions, the answer is lc, build a dedicated SI engine designed for natural gas combustion with an optimised compression ratio of 12-14:1. Even so, there are Iwo further possibilities: the engine will run equally well using lean-bum combustion (like a diesel), or stoichiometric combustion (like a petrol engine). But while a lean-burn engine can use an oxidation catalyst, going stoichiometric allows the use of a three-way catalyst. Iveco has invested much time and effort in assessing these Iwo routes and has concluded that if emissions reduction is your goal, then a stoichiometric air/fuel ratio is needed.
Diagram 1 shows how NOx, HC and CO vary with the air/fuel ratio (lambda), and the implications of lean-burn and stoichiometric solutions. For a stoichiometric engine (where lambda=1.0I NOx levels are high, at 209/kWh. But this (along with HC and CO levels) can be reduced by a three-way catalyst. For a lean-burn engine (lambda=1.6) NOx is already very low
(at 1.0 g/kWh) and the HC and CO can be oxidised by means of an oxidation catalyst. There appears to be little difference between the two, but Diagram 2 shows that an engine running at lambda=1.6 has very poor combustion stability compared with the same engine running at lambda=1.0. In other words, lean burn is fine for constant-speed running (for an electrical generator, say), but problematic in an automotive application where engine speed is constantly varying. Transient operation inevitably demands spontaneous enrichment, and consequently, increased NOx emission.
Methane is a very stable
3 molecule, and if it is left unburned during combustion 8 it can prove difficult to han
u die in an exhaust catalyst e However, stoichiometric run ning produces exhaust gas temperatures of around 550'C-high enough to achieve near-100% methane conversion. Lean bum results in significantly lower temperatures (around 360`C) at which point the conversion rate is below 20%, allowing unburned methane to reach the atmosphere. This is significant in envi ronmental terms as methane is 27 times more conducive to global warming than carbon dioxide.