TDi

TDI stands for "Turbo Direct Injection", which is the whole reason the motor is notable at all. It is both turbocharged and features rather advanced electronic fuel injection, which combine to deliver quite impressive performance from a 2-liter diesel engine. The most impressive statistic of the motor, though, is not its horsepower (a paltry 90, though it can be chipped and given simple intake and exhaust modifications to get better than 140), but rather its mileage - well over 50 miles/gallon if you drive it nicely.

Definitely an option worth an extra grand on your new VW.

The horsepower rating on the American-model 1.9 TDI may not be impressive, but the torque is much more so: 155 ft-lbs - more than an Acura Integra VTEC.

Furthermore, in Europe, the range of TDI models is much broader, with choices ranging up to the 180-hp, 2.5-liter diesel available in the Audi A6. Or, if you prefer economy, you can get a VW Lupo with a tiny 1.2 three-cylinder - the most fuel-efficient production car on Earth, at almost 90 miles per gallon.

The term TDi is a cause of minor confusion since there is no single accepted meaning for the acronym. The two most commonly used forms are "Turbo Direct Injection" and "Turbo Diesel Intercooled". The former has been rendered redundant since most, if not all of modern diesel engines are equipped with direct injection.

The European TDi Diesel range of VW turbodiesel engines has recently been extended to include two new models. Both use a system of individual unit high-pressure injectors, or pumpe duse in German, abbreviated to PD. The result is a slight increase in horse-power from 90 and 110 to 115 and 130 BHP respectively, and a whopping increase in torque to 210 ft/lb for both engines.

Traditionally diesel engines use a single fuel pump that distributes fuel to all injectors. The PD system uses a high-pressure pump for each injector. This allows for improved fuel economy since diesel-injection is controlled with unprecedented precision, resulting in a frugal 53MPG in combined cycles. It also improves performance by virtue of higher pressure and better controlled fuel injection.

All models are equipped with a Garrett variable-vane (variable-geometry) turbo that increases boost to respond to higher loads, and a Garrett waste-gate (known as dump-valve in the US). The turbocharger's vanes, unlike conventional turbos, can shift the vane geometry to provide a more forceful airflow when an increase in load is detected to provide the increase in power required.

The torque on the previous engine was already impressive but the recent increase has matched the performance of diesel hatch-backs to that of several GTI-spec petrol models. Driving the 110 BHP model TDi myself, I frequently encounter torque-steer when attempting to convince the front wheels to grip tarmac in second-gear at 2500 rpm and shudder to think of the way the new engines must respond to similar treatment unless some form of torque-limiter is provided as an option. The Acura (Honda in Europe) Integra equipped with a Limited Slip Differential (abbreviated in true Japanese fashion to LSD on the rear windscreen of the Integra!) fails to cope with the torque when shifting up at high rpm and as correctly stated in the previous writeup never develops torque in the range of the TDi Golf engine, so a more rapidly reacting system must be implemented to make the diesel models safe to drive in all but optimal conditions.

Increasing power and torque while increasing fuel economy sounds too good to be true, and those who advocated the switching to diesel as a source of less polluting, more economical fuel now add increased torque to their artillery of arguments in favour of diesel.