Hold Your Horses: Engine Limitations Explained!
July 28, 2018
|
By:
Jasper De Hertog
PREVIOUSNEXTLIST

Hold Your Horses! Engine Limitations Explained...

Last week it has been discussed that the IO-360-L2A proves to be a pretty powerful powerplant despite its compact shape. However, this does not mean its potential can always be exploited in an unrestricted way.

1. About Engine Power

We might be pretty familiar with the throttle lever as the black lever used to increase or reduce engine RPM and hence power output. However, how does that lever actually regulates engine power?

First we’ll refresh your mind on what engine power actually is. Basic mechanics suffice to prove that the power output of an engine is proportional to both engine speed and torque.

  • The engine speed is simply the speed at which the crankshaft runs and is indicated on the tachometer as revolutions per minute - RPM.
  • Torque on the other hand refers to the turning moment, or simply stated, the rotational force of the crankshaft. Being driven by the pistons. It is proportional to the forces exerted on the crankshaft by the connecting rods.

Hence the power output of the engine depends on both its speed and the forces exerted on the pistons during the combustion process.

Source: EuroPilot Center Airframes & Systems, Powerplants iBook

2. About the Throtle

Having explained the power concept you might start to wonder how the throttle is able to control the power output of the engine.

Looking in an English dictionary throttling is defined as “choking” or “strangling”. This is exactly what the throttle does to the engine …it controls the combustion process by choking or limiting the air supply  to the engine by means of the throttle valve located in the intake manifold or simply stated, the air inlet. 

Source: EuroPilot Center Airframes & Systems, Powerplants iBook

Whenever the opening of the throttle valve is reduced by retarding the throttle, the inflow of air into the intake manifold is restricted to a larger extent. Under the continuing movement of the pistons drawing air into the cylinders, the pressure hence density of the air in the intake manifold decreases. This causes a lower amount of oxygen particles to be available for combustion which will cause the intensity of the combustion to reduce.

Hence the forces exerted on the pistons and torque will decrease. In engines with a fixed pitch propeller, this will cause a subsequent reduction in engine speed or RPM.

Whenever the opening of the throttle valve is increased, the airflow into the intake manifold is less restricted. This will cause the pressure hence density of the air in the intake manifold to increase. This causes a higher amount of oxygen particles to be available for combustion which will cause the intensity of the combustion to increase. Hence the forces exerted on the pistons and torque will increase. In engines with a fixed pitch propeller, this will cause a subsequent increase in engine speed.

3. RPM Limitations

When referring to the IO-360-L2A operator’s manual and Lycoming’s Service Bulletin No. 369Q “Engine Inspection after Overspeed”, the maximum continuous RPM is stated as 2,700 RPM irrespective of altitude. Hence the hard RPM limit for the IO-360-L2A corresponds to 2,700 RPM.

In addition to this limit, Lycoming’s operator manual states:

On engines with manual mixture control, maintain mixture control in “Full Rich” position for rated take- off, climb, and maximum cruise powers (above approximately 75%).”

Hence it imposes a power limitation above which the engine should only be run using a fully rich mixture.  This so-called maximum leaned power setting is also reflected in the performance section of the Cessna 172 SP Pilot Operating Handbook stating:

“Maximum cruise power using recommended lean mixture is 75% MCP. … Operations above 75% MCP must use full rich mixture.”

When looking into the C172 SP’s performance tables, the maximum RPM corresponding to this maximum leaned 75% cruise power setting can easily be extracted.

Source: Cessna Pilot Information Manual Revision 2

As indicated in the performance tables, these RPM limitations depend on  the density altitude at which the engine is operating. Hence they will be affected by both pressure altitude and outside air temperature. On a standard day, the maximum RPM values corresponding to the maximum leaned 75% cruise power setting are:

  • 2,000 ft - 2,500 RPM
  • 4,000 ft - 2,550 RPM
  • 6,000 ft - 2,600 RPM
  • 8,000 ft - 2,650 RPM
  • 10,000 ft - 2,700 RPM
  • 12,000 ft - 2,700 RPM

These floating RPM limitations will be lower when temperatures are below standard and increase when temperatures are above standard.

4. Maintaining the RPM within limits

Spoken about the RPM limitations on the engine, you might start to wonder how this affects us as pilots.

A first important remark is that unlike most modern car engines, the IO-360-L2A is not protected against exceeding limiting engine parameters. The only restriction on engine RPM is sitting right between the yoke and the back of the pilot’s seat …so yes, it might be a good thing to know the limiting figures behind the throttle.

A first and obvious important consideration is never ever to allow the RPM to exceed the hard RPM limitation of 2,700 RPM. This limit is clearly indicated by the beginning of the red arc on the tachometer.

Furthermore the previously described floating RPM limitation should be taken into account. Does this mean the engine speed is not allowed to exceed this floating limit? …It is allowed. However, it is only allowed under a specific operating condition being that the engine should be running at a full rich mixture. This floating limit is clearly indicated by the beginning of the upper white arc on the tachometer.

When looking at the performance table, the RPM limitations throughout the aircraft’s operating range would appear as shown in the following table.

Source: Cessna Pilot Information Manual Revision 2

Being spoiled by the G1000 avionics suite, the upper white arc on the tachometer changes depending on altitude to reflect the floating RPM limitation.

Source Garmin G1000 Cessna NAVIII Manual

The actions to be taken in case of an overspeed event depend on the extent of the engine’s exposure to excessive speeds. Hence the difference between a momentary overspeed and an actual engine overspeed condition must be defined. Lycoming defines both conditions as follows:

  • Momentary overspeed is defined as an increase of no more than 10% of rated engine RPM for a period not exceeding 3 seconds. If the duration and amount of overspeed falls within the limitations defined as momentary, no further maintenance actions are necessary.
  • Engine overspeed, the engine operates above its rated RPM. Operation of an engine above its rated RPM can accelerate wear on already stressed components. The consequences of overspeed vary by engine type and model and depend upon several factors such as duration of overspeed as well as the amount of overspeed.

In the event of an actual engine overspeed condition, the event needs to be logged in the engine logbook. 

Whenever the maximum RPM has been exceeded by less than 5%, no specific inspections are required.

Whenever the maximum RPM has been exceeded by 5% to 10%, several inspections need to be performed. These include a check of the oil filter to assess internal wear of the engine. Furthermore the cylinders, valvetrain and magneto’s are subject to specific overspeed inspections.

Whenever the maximum RPM limit has been exceeded by more than 10%, the engine needs to be removed from the airframe and disassembled to check internal structural damage.

Obviously an overspeed event might cause undesirable downtime and extensive financial consequences.  So… always show good practice to hold your horses and keep a close eye on engine speed!

We'd love to hear from you!

Please post your comments/experiences in the comment section below.

Jasper De Hertog
Jasper started his aviation career at the age of 26 and is active as a full time flight instructor at EuroPilot and SoCal Pilot Center. He graduated from the Antwerp Maritime Academy in 2008 and has been working worldwide on various specialized ships. During his seagoing career he became passionate about aviation. In 2017 he docked for the last time and became fully dedicated to aviation training.
PREVIOUSNEXTLIST