**Q: Which is the better Focus petrol engine and why - the 108bhp 1.6 or the 115bhp 1.8?
Also, I've heard about diesel particulate filters and variable-geometry turbochargers - what are they and what do they do?
We much prefer the 1.6 to the 1.8. It's lighter, more economical, more refined and only a little slower than the more expensive 1.8.
A diesel particulate filter is there to absorb the small pieces of carbon that are present in diesel exhaust fumes.
Called PM10s (because the filters trap particles less than 10 microns in diameter), they are the largest particles that your nose and throat can't effectively filter, so they end up in your lungs.
There, they are thought to aggravate existing respiratory diseases and contribute to higher levels of asthma and heart disease.
A variable-geometry turbocharger is designed to provide power boost across a wider spread of an engine's rev range.
Turbochargers need to spin at a certain speed before they start to effectively force extra air into the engine and provide a power boost. As a result, fixed-geometry systems can work poorly at low engine speeds - leading to turbo lag - or force in too much at higher speeds, which can also cause a drop in power.
A variable-geometry system overcomes this either by altering the pitch of the vanes on the turbine itself or by moving the turbine along its shaft within its housing.