Small Diesels

As diesels have soared in popularity - in the past three years, sales have increased by nearly 1000 per cent - so have the myths and misconceptions about them.

Yes, they are more economical than equivalent petrol engines, but in small car/small engine territory the difference in consumption rates is not great.

That's because most of today's 1.3-2.0 litre petrol engines are also very economical, and in a relatively light, compact car the main advantages of diesel are less evident.

Move up to the other end of the scale, to big 4WD wagons, and diesel is the natural choice. A diesel engine's low revving, high torque characteristics are ideally suited to efficiently shifting a large mass - like a truck or a train - so a diesel 4WD can use 50 per cent less fuel than its petrol counterpart.

All of the small diesel cars here are European. They are among the most popular models in the EU, where diesel is taxed at a lower rate than petrol, and, in total vehicle numbers, diesel is actually the dominant engine type.

The petrol equivalents of these cars require premium unleaded, so pump prices are comparable. However, you have to factor in the higher purchase price - up to $5000 - of the diesel version over its petrol counterpart.

It can take many years and tanks of fuel to get that money back.

Diesels emit less carbon dioxide, the main greenhouse gas, but they also produce carcinogenic particulates - albeit at a fraction of previous levels.

Once you adjust your right foot technique, you can drive a diesel smoothly and quickly. Its running characteristics - low revs, strong torque, turbocharging - have charms and efficiencies of their own, especially on the highway where you can better 5.0 litre/100 km.

Diesels are an acquired taste, so it's important to take a lengthy test drive to experience their different performance characteristics.

BMW 120d

The 120d's 2.0 litre turbo diesel engine produces 115 kW of power at 4000 rpm and 330 Nm of torque at 2000 rpm. It's matched with a six speed sequential automatic.

Priced at $47,800, it costs $3000 more than the 120i petrol automatic which produces 110 kW/200 Nm.

The numbers give the diesel quicker acceleration: a claimed 8.5 seconds from 0-100km/h, compared with 9.2 seconds for the 120i.

BMW claims an average consumption of 6.6 litres/100 km for the 120d, against 7.9 litres/100 km for the petrol model - a minimal advantage for diesel, albeit here in a contest with one of the most fuel efficient petrol engines on the market.

We sampled the 120d on a brief drive in Victoria. It's a long legged device, which works smoothly with the automatic, but we were surprised that its performance did not feel quite as strong as the engine's outputs suggest.

It is also relatively noisy by current diesel standards, and some vibration is evident through the car at idle and under acceleration.

The 2.0 petrol engine, in contrast, is exceptionally smooth and quiet.

Otherwise, the 120d has all the strengths and weaknesses of other 1 Series BMWs. At $55,000+ on the road, any notion of "economy" is nonsense.

Fiat Punto

The Italian giant has returned to the Australian car market with the Punto, a beautifully styled small hatch, available with 1.3 litre (from $22,990) and 1.9 litre turbo diesel engines (from $27,990), plus a 1.4 litre petrol (from $19,990).

The 1.3 produces 66 kW of power at 4000 rpm and 200 Nm of torque at 1750 rpm, matched with a six speed manual gearbox.

When cold, it takes a few seconds to kick into life. Below 2000 rpm, it is very lethargic. Its best operating range is 2500-4500, where you're rewarded with decent acceleration and relaxed highway running, sufficient to hold 100 km/h on the cruise control - a feat beyond any small petrol engine.

The 1.3 Punto's ADR 81/01 combined average of just 4.6 litres/100 km makes it the most economical car on Australian roads, according to the federal government's Green Vehicle Guide.

In the real world, you'll still be an infrequent visitor to service stations. Expect to use 6-7 litres/100 km in town, and as little as 4.5 litres/100 km on the open road.

Dynamics are enjoyably sporty in flavour. The ride is surprisingly comfortable; coarse bitumen generates a lot of tyre noise.

Anti-lock brakes, air and six airbags are standard on the five door 1.3, which also scores a five star EuroNCAP rating.

The fragility of Fiats has inspired many jokes over the years, and it performs poorly in independent owner satisfaction surveys.

Holden Astra CTDi

The new Astra diesel is available in the five door hatch bodystyle only with a 1.9 litre engine: a 110 kW (plus 320 Nm of torque) 16 valve twin cam/six speed manual, priced at $29,990, or an 88 kW/280 Nm eight valve single cam/six speed sequential automatic, at $31,490.

These engines are actually made by Fiat, and are variants of the 1.9 found in the Punto.

They are common rail engines, with up to the minute technology including variable vane turbochargers and ultrafine, phased injection sequencing for smoothness and economy, called Multijet.

Holden claims an ADR 81/01 test average of 6.0 litres/100 km for the manual and 7.4 litres/100 km for the automatic. In comparison, the 90 kW/170 Nm petrol Astra's numbers are 7.8 and 8.2 litres/100 km respectively.

So it's going to take a while to recoup the diesel's $4500 (auto) to $5000 (manual) price over the CDX petrol Astra - as in 12.8 (manual) to 25.8 (auto) years, assuming a pump price of $1.45 per litre for both fuels and an average annual distance travelled of 15,000 km. However the CDTi does include Traction Control (TC) and Electronic Stability Program (ESP) which are not availble on the CD or CDX petrol Astras.

The Astra diesel's performance, however, is superior to its underdone petrol counterpart. Its huge torque advantage makes the petrol engine feel relatively weak, especially in a contest between automatics, where the diesel's extra two ratios amplify the engine's strengths.

Secure handling, a firm yet compliant ride, five star safety, plenty of space and stability control add to the Astra's appeal. It's a bit noisy, though, especially the eight valver.

Peugeot 307 HDi

Peugeot uses 80 kW/240 Nm 1.6/five speed manual and 100 kW/320 Nm 2.0/six speed manual diesel drivetrains in the 307 five door hatch and the Touring wagon. No automatic is available.

The 1.6 HDi costs $29,490; the 2.0 HDi is $32,290.

These prices are $4500 (1.6) and $3300 (2.0) higher than the respective petrol hatchbacks. The Touring wagon costs $31,190 (1.6) or $33,990 (2.0), the latter also carrying a $3300 premium for the diesel.

Peugeot quotes fuel consumption figures for the 1.6 diesel of 6.1 litres/100 km in town (vs 10 litres/100 km for the 1.6 petrol) and 4.3 litres/100 km on the highway, where the petrol engine returns 5.8 litres/100 km.

The 1.6 is a sweet engine, albeit a leisurely one. Its 0-100 km/h time is comparable with its petrol counterpart: 12.2 seconds, as against 11.9 seconds. Its ability to stay in fifth all day on the highway is remarkable for an engine of this size.

Comfortable seats, an absorbent open road ride and the extra space of the Touring are other pluses: Peugeot's troubled quality and reliability record and a tight back seat are disincentives. Four star safety rating.

The 1.6 diesel is also available in the beautiful Citroen C4, from $29,990.

VW Golf TDi

VW offers two turbo diesel engines in the Golf, priced from $29,990.

The base model Golf 1.9 litre turbo diesel produces 77 kW of power at 4000 rpm and 250 Nm of torque at 1900 rpm.

A five speed manual is standard; VW's six speed sequential manual direct shift gearbox (DSG) costs an extra $2300.

VW employs a different fuel injection system, called Pump Duse (pump injectors), to other manufacturers. Instead of fuel reaching the injectors via a single pressurised reservoir (called a common rail), a plunger pump in each injector delivers the fuel at up to 50 per cent higher pressures than in a common rail.

Pump Duse is claimed to convert up to 43 per cent of the thermal energy in fuel into mechanical energy, a greater percentage than for any other liquid fuelled production car. 

VW (and sister brand Audi) sets the standard for small diesel engines. Performance, refinement and economy are excellent. If you prefer an automatic, the DSG transmission is the best in the class.

Fuel consumption is claimed 5.8 litres/100 km, less than half a litre more than the six speed manual. Acceleration is also comparable: a claimed 11.2 seconds to 100 km/h for the auto, only one tenth of a second slower than the manual.

The 2.0 litre, which produces 103kW/320Nm costs $32,490, $2500 more than the 110 kW/200 Nm 2.0 direct injection petrol equivalent, in Comfortline specification.

VW claims ADR 81/01 average fuel consumption of 6.1 litres/100 km for the 2.0 DSG diesel, against 8.5 litres/100 km for the 2.0 petrol six speed automatic.

The 2.0 diesel auto Golf is actually quicker than its petrol counterpart. The 2.0 petrol engine, though exceptionally smooth and tractable, also requires 98 octane unleaded, which often costs more than diesel.

The Golf scores five EuroNCAP stars, rides comfortably and handles securely. Anecdotal evidence suggests that quality and reliability issues are still causing some owners grief.

VW Polo TDi

The Polo TDi, priced at $22,990, uses the same 1.9 turbo diesel engine as the base model Golf, with slightly lower outputs - 74 kW and 240 Nm - and a five speed manual gearbox.

It gives the smaller, lighter Polo TDi remarkably muscular performance from 2000 rpm to 4500 rpm, where no petrol powered small car can touch it. The 0-100 km/h trip takes 10.1 seconds.

VW claims an average 5.0 litres/100 km. We achieved high fours on a gentle country run; we averaged 5-5.6 litres/100 km on a variety of rural roads, and 7-8 litres/100 km in town.

The Polo is as solid as a small tank and very comfortable, but it's noticeably front heavy, so dynamics are a bit doughy.

An alarm, cruise control, anti-lock brakes and power windows are standard. Only two airbags are provided; the Polo scores four NCAP stars.

By Bill McKinnon, August 2006.