Q) Why do some manufacturers (such as Toyota) implement a short 10 000 km service interval on some models, and other vehicles have 15 000-20 000 km intervals (or even longer)? Does the length of service intervals say something about the quality of the vehicle?
A) Powertrain engineers establish service intervals during development of a certain product by taking into account usage profile and application. They run specific durability tests – where the oil is analysed – and also extract oil samples during the engine dyno and fleet-vehicle programmes to determine the level of oil contamination.
Many modern vehicles run an oil-quality algorithm in the ECU that is calibrated to determine when an oil change is necessary. The algorithm uses engine speed, load and temperature to predict the degradation of the oil. The algorithm also has a maximum-mileage threshold at which the oil has to be changed.
A manufacturer such as Toyota has historically been conservative with oil-change intervals to preserve its legacy of reliability. We believe there is little correlation between vehicle quality and length of service intervals, although it takes engineering expertise to calibrate an oil-change algorithm effectively.
Q) I recently bought an imported 1,8-litre (ABS) Golf engine fitted with a carburettor. I see there is a difference in power and torque (4 kW and 5 N.m more) although bore and stroke are the same. Why the difference in the power and torque outputs?
A) Without knowing the exact details of the two engines, but given that they have similar capacity (including bore, stroke and, we assume, compression ratio), we can offer general comments. In a naturally aspirated petrol engine, the air-fuel ratio is normally close to 14:1 (or lambda one), which is the ideal combustion ratio.
The power the engine produces comes from the chemical energy in the fuel. If both engines are equally efficient, any additional fuel that is burned will lead to more power (and torque) in that specific engine. The amount of fuel that can be burned is mostly limited to the amount of air flowing through each engine (at the 14:1 ratio, as explained).
Therefore, the engine that produces more power and torque probably “breathes better”. A possible reason is differences in intake/exhaust design, valve timing and duration (different camshaft design) or higher engine speeds. A combination of the aforementioned could also be a reason. What’s more, similar engines from manufacturers are often tuned to the specific requirements of respective markets.
