For simplicity we're going to ignore aerodynamics and vehicle mass for a moment...

Are small (around 1 litre) car engines designed to tolerate a higher duty cycle than large (2+ litre) car engines?

Cars are usually all travelling at roughly the same speed which means the engines will be putting out proportionally similar amount of power, however a small engine would be operating at a greater proportion of its output than a large one to maintain the same speed.

For some background to this, I have a 1.1 l car and I spend quite a lot of time with my foot completely flat on the floor, especially going uphill on a high speed road (60 or 70 mph limit) to keep up with the traffic. I think my record's somewhere around 2 minutes on full throttle.

  • $\begingroup$ What dues duty cycle have to do with what you're asking about. You mention duty cycle in the title and again in the body, but the description makes it sound like you're really asking about the fraction of full power the engine is operating at. Huh? $\endgroup$ – Olin Lathrop Dec 16 '15 at 13:03
  • $\begingroup$ Good point. Further googling suggests that I've managed to mangle duty cycle and drive cycle into some other measurement that doesn't exist. I s'pose the question could be rephrased as 'Would a small car engine last longer running at a high (or full) throttle setting than a larger, higher output car engine?' $\endgroup$ – squigbobble Dec 16 '15 at 13:17

Most (modern) small and large car engines are designed for 100% duty cycle. This means that at 100% rated power(gas pedal all the way down) the engine can run continuously. Heat dissipation is the limiting factor like Dave Tweed stated. Cars that are not designed to continuously dissipate 100% of the heat generated at max power require the driver to watch the temperature gauge to limit the power use.

Modern engines do not have this problem because the engine is governed (speed regulated) below the cooling capacity of the radiator. Most modern engines use electric fans on the radiators that are independent of engine rpm; greatly increasing continuous cooling capacity.

Older cars and "high performance" cars may have power that exceeds the cooling capacity. Any engine that has had the maximum engine speed regulation removed or any engine that can be "red lined" can also overheat. An engine boosting system such as nitrous oxide also exceeds the cooling capacity and thus must be used intermittently.

You will often see both large and small cars pulled over for overheating along a steep hill on a hot day. In this instance the "duty cycle" under these operating conditions was not continuous (100%). However, duty cycle is typically not used to describe this behavior, because it is a design expectation that it can operate continuously. The engine was simply operating outside of its designed range.

Duty cycle is not influenced by the size of the engine, but rather, duty cycle is a design parameter when designing an engine system. Most cars would be designed for continuous duty, while race cars would be designed for intermittent.

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The primary issue (ignoring secondary issues like internal wear) is whether or not the engine can get rid of the waste heat.

Assuming that all engines for a given fuel have roughly the same overall thermal efficiency, for a given power output, a certain amount of waste heat needs to be dissipated regardless of the physical size of the engine.

The heat is dissipated through the radiator, and if it is effective, then for any given power level, the coolant temperature will be stable. If the radiator is being driven beyond its capacity, the coolant temperature will keep rising.

So, the bottom line is this: When operating at full throttle, does the coolant temperature stabilize at a safe level? If not, you should consider getting a different car for that kind of driving.

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  • $\begingroup$ The coolant temp (according to the gauge) never rises above the warmed up temperature, although if I slow down suddenly after caning the car I get a blast of noticeably warmer air from the heater. $\endgroup$ – squigbobble Dec 14 '15 at 15:52

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