Engineering of Water Systems

Engines: Intake and Exhaust Valves, Cooling Systems, Lubrication, and More.

By Ed Butts, PE, CPI

This edition of The Water Works represents the final installment on internal combustion engines for use in standby and backup service to water systems.

So far, we have reviewed the concepts of standby and backup equipment, introduced the basics of engines, and discussed displacement (bore and stroke), fuel consumption, and cylinder pressure. This month we wrap up the series with an examination of intake and exhaust valves, parasitic losses, lubrication and cooling systems, and engine performance curves.

Intake and Exhaust Valves

Engine valves are check valve-type devices used in internal combustion engines to allow or stop the flow of a fluid-air mixture or air alone into or expelled as gas from cylinders or combustion chambers during the engine while it is operating.

Engine valves are commonly employed in every type of internal combustion engine including gasoline, diesel, kerosene, natural gas, or propane. In a typical engine, two engine valves are used for each cylinder: an inlet (or intake) valve and an exhaust valve (Figure 1).

Engine valves are mechanically opened and closed due to repeating action from a camshaft. The intake or exhaust valves are in the head casting of the engine within the intake or exhaust manifolds, respectively. The area where the valve seals against the head is called the valve seat. Most medium-sized internal combustion (IC) engines have either intake ports and exhaust valves, or both intake and exhaust valves.

Smaller two-stroke gasoline engines use intake and exhaust ports (i.e., no valves). Some four-stroke engines utilize two suction inlet valves and two exhaust discharge valves. The suction valves supply ambient air to the combustion space while the discharge valves discharge exhaust gases to the atmosphere.

Fresh air (intake) valves are larger than the exhaust valves. This ensures fresh air replaces as much of the exhaust gases as possible within the combustion space when the cylinder is recharged. The air or fuel-air mixture is admitted into the cylinder by the inlet valve. When closed, both valves seal the combustion space tightly during combustion preceding the power stroke.

The valves are usually made of austenitic stainless steel, a corrosion and heat-resisting material. The inlet valve is subjected to less heat, so it is usually made of nickel-chromium alloy steel. The burned gases escape through the exhaust valve. The exhaust valve is usually made of sili-chrome steel which is an alloy of silicon and chromium with an unusually high resistance to heat.

The angular face is ground on the valve head to make an angle of 45° or 30° to match the angle of the valve seat in the cylinder head. Spring retainer lock grooves are provided at the end of the valve stem.

Engine Intake Airflow Calculations

The air intake rate, in cubic feet per minute (CFM), is generally available from the engine manufacturer. If CFM specifications are not available, use the volumetric efficiency calculation method below.

Two simple rule-of-thumb calculations for estimating the required intake airflow (CFM) is to use 2.50 CFM per HP or HP/2.50 = Intake CFM. For the estimated exhaust airflow (CFM), you can multiply the HP rating by 2 or Exhaust CFM/2 = HP.

 

 

 

 

 

 

For Estimating Volumetric Efficiency:
4-cycle gas engine-naturally aspirated = Use 0.70-0.80
2- and 4-cycle diesel engine-naturally aspirated = Use 0.90
Diesel engine-turbocharged = Use 1.50-3.00
Note: Engine volumetric efficiency ratings are best obtained from the engine manufacturer.
Equation 2a: Engine Intake Airflow from Horsepower:
Airflow (CFM) = (Horsepower × 0.625) × 1.1104
Equation 2b: Required Airflow (CFM): 2.745 × HP × Engine’s BSFCs

Engine Frictional and Parasitic Losses

An engine has many moving parts that produce frictional forces and resistance to movement. Some of these friction forces remain constant throughout the engine’s operating range so long as the applied load is also constant. Some of these friction losses increase as the engine speed increases, such as piston side (lateral) forces and connecting rod and bearing forces due to increased inertial forces from the oscillating
piston to the crankshaft.

A few friction forces actually decrease at higher speed, such as the friction force on the camshaft’s lobes used to operate the inlet and exhaust valves (the valve inertia at high speed tends to pull the cam follower away from the cam lobe).

Along with friction forces, an operating engine has inherent pumping losses, which is the work required to move air into and out of the cylinders. This pumping loss is minimal at low speed, but increases at approximately the square of the speed, until at rated power an engine is using about 20% of its total power production to overcome the friction and pumping losses.

In addition to frictional losses in engines, most engines and power plants also lose a measure of power through parasitic losses. Parasitic losses are not necessarily frictional losses, as they are generally independent from the required and actual functions of the engine but are generally regarded as necessary for the proper and safe operation of the engine. Parasitic losses often vary between engine types and styles but typically include all accessories and ancillary belt-driven devices, including the engine’s generator/alternator, fan, fuel and water pump, and other loads such as belt/pulley combinations needed to drive a super or turbocharger and hydraulic pumps.

Parasitic loads are not regarded as automatic engine loads, as some engines that use water jacket cooling do not consequently require an integral fan or water pump for engine cooling, so these loads can usually be ignored. For purposes of accuracy, parasitic loads should be examined on an as present basis and added individually to determine the total impact to the engine’s output power. Even with this variance, in the absence of specific engine performance data that includes the power loss from these components, it can usually be safely
assumed that the typical stationary engine will consume between 5%-20% of the rated horsepower for parasitic loads.

In total, the combined impact from engine frictional and parasitic losses will generally amount to between 10%-20% at minimum speed up to 25%-30% of total engine performance at the engine’s rated speed and power output. As a rule of thumb, an assumed total loss of 25% will generally provide an adequate margin of safety.

Additional Derating Losses

In addition to the frictional and parasitic losses in an engine, additional derating or corrections for specific conditions that are not included in the original design must often be included as a HP deduction from the standard conditions the engine was designed for. These include factors such as elevation, air temperature, humidity, and other possible atmospheric and environmental factors that can have a direct influence on engine performance, including the following derating:

• Altitude: Generally, engines and power plants are rated for Mean Sea Level (MSL) performance up to 500 feet of elevation. Installations above 500 feet MSL require a derating of 2% for every 1000 feet of increasing altitude. Thus, an engine operating at 2000 feet elevation will require a 4% power derate for increased altitude.
• Ambient Temperature: Since the combustion and cooling qualities of air decline with increasing temperature, it may also be necessary to reduce an engine’s power for a higher ambient temperature (Table 1).

In addition to derating required for altitude and ambient temperature, the method of transmitting power from the engine to the driven device, usually consisting of a pump or generator, must be included. For the most common forms of power transmission, the derating percentages in Table 2 are often used.

Engine Cooling

Regardless of the fuel or type of engine in use, only 20% to 40% of the BTU combustion value of the fuel can be converted directly to shaft (output) power; the remaining 60% to 80% of the fuel’s value is generally converted to a single component: heat. This heat must be carried away from the engine using either air or water cooling to prevent eventual overheating conditions eventually followed by seizing.

Air cooling utilizes a fan and radiant cooling that forces ambient air over the engine to carry excess heat away from the engine. Air-cooled engines (Figure 2) are popularly used in horsepower of 50 HP (37.3 kW) and less, although they are available in multiple hundreds of HP sizes. Air-cooled engines are commonly used as stationary or portable booster pump or generator engines for water pumping or center pivot/linear applications.

The fact that an air-cooled engine needs no radiator, water jacket, or water pump or many of the other components associated with water cooling means that the engine complexity and function are simpler, and that means the manufacturing costs and engine prices can be lower.

Air-cooled engines are normally made in a horizontally opposed or wide V configuration. The cylinder head and cylinders or barrels of an air-cooled engine are cast with cooling fins. These fins radiate and distribute the heat of the engine over a wide area.

The shape and size of both fins and the fan are crucial to the engine’s efficiency as is the spacing of the fins. Wide spacing of the fins will provide easier airflow, and therefore impose only a small load on the fan, which can be quite small. Closer spacing of the fins will extract more heat but will also demand a more powerful fan to maintain the cooling process, which in turn will absorb more of the engine’s power due to the higher parasitic loss.

The cooling fan is positioned in a semicircular ducting, with the ducting also covering the cylinder head. Its interior is fitted with baffles which channel the flow of air over the engine cooling fins and through an oil cooler.

Water cooling uses one of three types of cooling systems. The first, a conventional radiator/fan assembly (Figure 3), uses an engine-driven water pump that rotates cooling fluid (usually a mixture of water and antifreeze) in a closed-loop circuit that flows around the engine block and head to collect and remove the generated heat away from these components. The higher temperature coolant is then delivered to
an onboard radiator where the heat is dissipated using an engine-driven fan that blows cooler air across the radiator core and away from the engine. The coolant fluid is then circulated back to the engine to collect and remove additional engine heat.

The second method of water cooling uses the same radiator and water pump principle, but in this case, the radiator, fan (if used), and water pump are separate components located often several feet from the engine. This allows the removal of heat through the use of cooling towers, heat exchangers, and other systems that would be impractical mounted to the engine.

The third method utilizes a principle often referred to as water jacket cooling (Figure 4 for an inline six-cylinder engine). With this method, a source of coolant fluid, generally potable or non-potable cold water at an ambient temperature of 60°-70°F, is used to circulate the fluid around the engine to cool it and carry heat away from engine components. The fluid is then either discharged to an external discharge point, such as a sump, or sent to a separate radiator or heat exchanger to remove the heat.

When a radiator is used, the implied heat is dissipated using a fan that blows cool air across a large surface area. Make-up water is sent into the system from the cold water supply to replace any loss of coolant, usually through a solenoid valve powered from a high temperature switch or basin float switch when a tank is used. This type of cooling system is applicable to well pumps where a small portion of the pumped water can be diverted and used for cooling flow.

The second and third cooling system are often advantageous in that they provide flexibility to the designer in lowering the engine’s noise level and also remove the parasitic loads of the water pump and fan from the engine loads. In some cases, these two loads can generate as much as 10% to 15% of the engine’s HP output.

In all cases, knowledge of the heat rejected from the engine and generator, or other driven machine, must be factored in the design and removed from the engine. Even if one of these methods are used for engine cooling, the engine and driven device will still generate radiant heat from the mechanical processes, friction, and loss of efficiency. This radiant heat must always be removed from the engine environment or room.

The engine-generated radiant heat (i.e., heat rejection to the atmosphere) is routinely provided with published engine technical data. Values are typically nominal with their specific tolerance noted. An adequate safety factor should always be added before using this published data in calculations.

Also known as water jacket heaters and engine preheaters, engine block heaters are essential for maintaining ideal starting temperatures for diesel and natural gas engines. NFPA regulation 110 requires all critical mission standby generators to be able to start and reach operating conditions within 10 seconds of power loss. This requires the engine coolant to be maintained at 100° to 120°F to avoid a potentially dangerous cold start. This is a mandatory item for all engines that operate fire pumps or automatic starting backup pumps or generators.

As important to observing adequate engine cooling procedures during operation is, ensuring adequate cooldown periods during and before the engine shutdown phase is equally important. A period of reduced engine speed and load, generally 3 to 10 minutes, must be implemented at the end of the engine operating period but before the engine shuts completely down to allow the engine components an opportunity to cool to a lower temperature. This is critically vital for engines with turbochargers as the devices can seize if not adequately cooled before engine shutdown.

Engine Lubrication

Lubrication is another important element of the proper operation of an internal combustion engine. The frictional loads generated through the constant sliding action of the pistons within the cylinders, opening and closing of the intake and exhaust valves, various bearings located throughout the engine, and the other forces resulting from the interaction of metal against metal result in severe friction and heat.

This friction must be relieved to allow the metal contact surfaces within the engine to continue to slide across each other in as much of a frictionless state as possible. Lubrication is provided by using the proper weight of a motor oil. Motor oil is available in various Society of Automotive Engineers (SAE) grades and weights to provide the proper balance of providing lubrication to all metal to metal surfaces; maintaining rotation throughout the engine with a minimal amount of internal adhesion to the metal surfaces; ability to dislodge, absorb, and transfer grit, soot, and metal shavings from engine components for removal in an oil filter; and ability to handle the tremendous internal engine heat without breaking down.

Although most motor oils are a derivation from crude oil, synthetic oils are now used in many engines that offer superior heat dissipation and lubrication benefits. In all cases, engine oil must comply with the manufacturers’ recommended grades and weights.

As with water jacket heaters, there are applications where engines must start and be ready within 10 seconds or less. When an engine is shut down, the lube oil on and in the internal components drains down to the oil pan. When starting again, the cold oil in the pan imposes a higher load on the engine before the operating temperature is reached. This horsepower demand stresses each of the components and accelerates the internal wear. Oil preheaters are available that will keep the oil at the proper temperature needed for rapid start. Examples of lubrication systems for most internal combustion engines are shown in Figure 5.

Engine Performance Curves and Ratings

As with water pumps, performance curves are also published for most diesel and gasoline engines in current production. Where curves for water pumps demonstrate peak or best efficiency at the pump’s optimum capacity rating, engines are rated for delivered horsepower and torque using the same type of X and Y axis curve.

In the case of an engine performance curve, however, the X is the horizontal axis and primarily used to indicate engine speed in RPM and the Y is the vertical axis displaying the output horsepower in HP or kW, torque in ft.-lbs. or Newtons, and fuel consumption in lbm per HP-hr or grams/kWh. Brake horsepower (BHP) is generally displayed in continuous and intermittent ratings.

In some cases, certain power plants will also display power ratings for short-term overload ratings as well as continuous and intermittent ratings. Unlike most pump curves, engine performance curves usually display another important value of engine performance, such as fuel consumption. In other cases, engine performance curves may also show noise or sound levels in decibels, emission levels in various units, or
oil consumption in gallons per hundred hours of operation.

Referring to a typical type of engine performance curve in Figure 6, it is apparent that the peak intermittent torque of 484 ft.-lb. occurs at approximately 1400 RPM, while the lowest fuel consumption of 0.348 lbm./HP-hr. occurs between 1600- 1800 RPM. Therefore, the expected fuel consumption with continuous loading at 1800 RPM would equal 0.348 lbm/HPhr. × 140 BHP = 48.72 lbm/hr./7.17 lb. for No. 2 diesel fuel = 6.79 gallons per hour. This speed range coincides with a midspan continuous power output of 140 BHP and an intermittent
power output of 160 BHP.

Typically, the lowest region or point on a fuel consumption curve or the peak torque comprises the sweet spot or best efficiency point for an engine selection. For this particular example, a continuous service load of 125-130 BHP or an intermittent load of 140-145 BHP at 1800 RPM would work well for this engine. This is a common rotational speed for many generators and pumps.

I usually avoid loading an engine up to the maximum continuous rating of an engine as I prefer to provide a reserve of around 10%-20% of a power allowance for the engine’s parasitic loads, future wear, fuel supply variances and filter losses, atmospheric and ambient temperature variances, and manufacturer’s tolerances.

Five industrial (IND) service ratings are available for engine (power units) power and performance. Typically, an engine manufacturer builds an engine for the applicable rating’s type of service and load duration expected from the engine. As opposed to generators, engine load ratings are usually based on either continuous or intermittent loads with variations, including short-term overloads.

Generally, engine performance curves indicate one or both ratings as a design guideline to enable determination of the horsepower and torque output. Currently, diesel and gasoline engines are service rated in accordance with ISO3046/1 for five different conditions: IND-A: Continuous Service, IND-B: Cycling, IND-C: Intermittent Duty, IND-D: Short Duration Overload, and IND-E: Emergency Service.

Each rating applies to the engine’s performance and power delivery as a power unit and is irrespective of the separate service rating placed on a generator. Each rating is outlined as follows:

1. IND-A (Continuous Duty): Continuous heavy-duty service where the engine is operated at maximum power delivery and full speed up to 100% of the time without load interruption or cycling.
2. IND-B: For service where loads or speed are cyclic (operating time at full load shall not exceed 80%).
3. IND-C (Intermittent Duty): Intermittent service where the maximum power output or speed are cyclic (operating time at full load shall not exceed 50%).
4. IND-D: For service conditions where maximum power is required for periodic overloads (operating time at full load shall not exceed 10% of the duty cycle).
5. IND-E: For service where the engine’s maximum power is required for a short time period or 5 to 10 second burst needed for initial motor starting or sudden overload. For emergency service, where standard or utility power is unavailable (operating time at full load shall not exceed 5% of the duty cycle).

Turbocharging, Supercharging, and Aftercooling

The previous formulas and examples were based on standard conditions and typical horsepower output based on the type of engine and fuel. Modifications and additions to many engines, primarily diesel engines, can extract additional horsepower and torque from naturally aspirated engines without appreciably changing the inherent characteristics of the engine itself.

These modifications, usually in the form of a turbocharger or supercharger, can increase the net shaft HP by up to 50% in some cases. A turbocharger, often simply called a turbo, is a turbine-driven forced air induction device that increases the efficiency of an internal combustion engine and resultant power output by forcing additional air into the combustion chamber.

Turbos and superchargers are basically modified air compressors that shove more air into an engine than the pistons would be able to pull in via suction on their own. Since more air equates to more oxygen, this means that the engine can efficiently burn more fuel and develop more resultant output power.

However, based on the limitations of thermodynamic laws, compressing this air also causes an increase in the temperature of the air, reducing the density of the available oxygen and increasing the odds of premature detonation in the cylinder. So, although the engine does develop more output horsepower using the same engine displacement, eventually there will be a point of diminishing returns where the boost pressure is concerned.

The general rule of thumb for a turbocharger is that a turbo will increase horsepower by about 7% (× 1.07) per pound of boost over a naturally aspirated configuration and a supercharger will increase it by 5% to 6% (× 1.06) per pound of boost, not accounting for temperature-induced power losses.

The supercharger’s HP increase is slightly lower because it requires power from the crankshaft to turn the compressor, where a turbo does not. For example, if an engine develops 150 horsepower (naturally aspirated), then an estimated additional 10.5 horsepower (7%) per pound of boost will occur with a turbocharger and 7.5 to 9 (5%-6%) additional HP for a supercharger. If the engine charger is developing 8 psi of boosted pressure, that results in approximately 234 output horsepower (approximately 36% increase) with a turbocharger and 210 to 222 output horsepower (approximately 28%-32% increase) with a supercharger. They are applicable for and used on both diesel and Otto (conventional gas) cycle engines from 30 HP to over 1000 HP.

Superchargers operate using a drive belt from the crankshaft, while turbochargers receive their drive power from diverted high-pressure exhaust gases. Other than the method of drive, they are essentially identical in terms of function.

This additional airflow and resulting horsepower increase also results in greater engine and component heat which must be adequately dealt with and removed from the engine during operation—particularly before total engine shutdown and from stationary engines and their enclosing environment where natural cooling air transference over the engine during transport is not present as it is with a moving vehicle.

A period of up to 10 minutes of reduced speed may be required to allow adequate cooldown of a turbocharger before the engine is shut down. In many cases, the use of water jacket cooling or auxiliary forced air fans to force ambient air across the engine and out of the building or enclosure is warranted.

Compared to a mechanically driven supercharger, turbochargers tend to be more efficient but less responsive in acceleration. A typical turbocharger consists of an inline radial flow turbine (axial flow turbines are used on larger diesel engines) and a centrifugal compressor, driven by the turbine, to boost air into the engine.

For stationary engines, such as standby generators and other power units, turbochargers must be applied carefully to ensure adequate exhaust gas volume and velocity is available to spin the turbine and compressor.

Engines exposed to prolonged periods of idling or wide ranges of speed variations may not receive the inherent benefits from turbocharging. Turbochargers start to produce an air boost only when a certain amount of kinetic energy is present in the exhaust gases. Without adequate exhaust gas flow to sufficiently spin the turbine blades, the turbocharger cannot produce the necessary force to compress air into the engine.

The operating speed (RPM) at which there is enough exhaust gas momentum to compress the air going into the engine is the boost threshold RPM which varies between engines. The boost threshold is determined by the engine displacement, engine RPM, throttle opening, and the size of the turbocharger.

Turbochargers can be a valuable tool and cost savings for marginally sized engine applications where a naturally aspirated engine may need eight cylinders to perform a task. For example, where a turbocharged six-cylinder engine will work at the same speed and horsepower as an eight-cylinder, naturally aspirated engine.

In many instances, turbochargers can be retrofitted and added to existing engines, preventing the added expense and hassle associated with changing out an existing engine when higher horsepower is needed.

In certain instances, especially in warm environments with stationary engines, the ambient air or air boosted from a turbocharger is often too warm to effectively aid in combustion. In these circumstances, the use of an aftercooler is often indicated.

An aftercooler (also called an innercooler in many cases) removes heat from the compressed air delivered from a turbocharger before it is returned to enter an engine’s combustion system. By precooling the air, this increases the engine’s efficiency and power by increasing the density of the charged air, resulting in far more air (oxygen) molecules per cubic foot of air.

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This wraps up this column and this series on engines. We’ll begin a multi-part series on generators in January.

Until then, keep them pumping!

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Ed Butts, PE, CPI, is the chief engineer at 4B Engineering & Consulting, Salem, Oregon. He has more than 40 years of experience in the water well business, specializing in engineering and business management. He can be reached at epbpe@juno.com.