Nem puts a spark into gas
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While German politicians are exercised by NOx emissions, British politicians get most heated over particulates, so between them they are pushing for exceptionally clean enginesparticularly for use in cities. iveco claims that its latest spark-ignition (SI) engine, using current technology and running on compressed natural gas (CNG), can reach the very low emission levels that are being proposed.
by Gibb Grace
• The fuel an engine uses significantly affects the emissions it produces. Hydrogen gas contains no carbon, so when burnt emits only harmless water vapour-but it is prohibitively expensive to produce. The next best candidate is natural gas; this contains 85-90% methane (CH4) which, in turn, contains 25% hydrogen by mass. This compares with 17.4% for LPG, and just 13.4% for petrol and diesel.
Methane also has a very high octane number of 130: when used with a high compression ratio (around 13:1), engine efficiency can reach 36%-down on the diesel's 40-45%, but better than the 30-34% of a petrol engine. Also, natural gas mixes easily with air to give a good homogeneous fuel/air mixture which helps complete combustion, and allows tight control of the air/fuel ratio: this is particularly beneficial during transient operation. On the downside, natural gas has a low specific energy compared with diesel, which reduces volumetric efficiency and poses storage problems.