In thermodynamics and engineering, a heat engine is a system that converts heat or thermal energy—and chemical energy—to mechanical energy, which can be used to do mechanical work. It does this by bringing a working substance from a higher state temperature to a lower state temperature. A heat source generates thermal energy that brings the working substance to the high temperature state; the working substance generates work in the working body of the engine while transferring heat to the colder sink until it reaches a low temperature state. During this process some of the thermal energy is converted into work by exploiting the properties of the working substance; the working substance can be any system with a non-zero heat capacity, but it is a gas or liquid. During this process, some heat is lost to the surroundings and is not converted to work; some energy is unusable because of friction and drag. In general an engine converts energy to mechanical work. Heat engines distinguish themselves from other types of engines by the fact that their efficiency is fundamentally limited by Carnot's theorem.

Although this efficiency limitation can be a drawback, an advantage of heat engines is that most forms of energy can be converted to heat by processes like exothermic reactions, absorption of light or energetic particles, friction and resistance. Since the heat source that supplies thermal energy to the engine can thus be powered by any kind of energy, heat engines cover a wide range of applications. Heat engines are confused with the cycles they attempt to implement; the term "engine" is used for a physical device and "cycle" for the models. In thermodynamics, heat engines are modeled using a standard engineering model such as the Otto cycle; the theoretical model can be refined and augmented with actual data from an operating engine, using tools such as an indicator diagram. Since few actual implementations of heat engines match their underlying thermodynamic cycles, one could say that a thermodynamic cycle is an ideal case of a mechanical engine. In any case understanding an engine and its efficiency requires a good understanding of the theoretical model, the practical nuances of an actual mechanical engine and the discrepancies between the two.

In general terms, the larger the difference in temperature between the hot source and the cold sink, the larger is the potential thermal efficiency of the cycle. On Earth, the cold side of any heat engine is limited to being close to the ambient temperature of the environment, or not much lower than 300 Kelvin, so most efforts to improve the thermodynamic efficiencies of various heat engines focus on increasing the temperature of the source, within material limits; the maximum theoretical efficiency of a heat engine is equal to the temperature difference between the hot and cold ends divided by the temperature at the hot end, each expressed in absolute temperature. The efficiency of various heat engines proposed or used today has a large range: 3% for the ocean thermal energy conversion ocean power proposal 25% for most automotive gasoline engines 49% for a supercritical coal-fired power station such as the Avedøre Power Station 60% for a steam-cooled combined cycle gas turbineThe efficiency of these processes is proportional to the temperature drop across them.

Significant energy may be consumed by auxiliary equipment, such as pumps, which reduces efficiency. It is important to note that although some cycles have a typical combustion location, they can be implemented with the other. For example, John Ericsson developed an external heated engine running on a cycle much like the earlier Diesel cycle. In addition, externally heated engines can be implemented in open or closed cycles. Everyday examples of heat engines include the thermal power station, internal combustion engine and steam locomotive. All of these heat engines are powered by the expansion of heated gases. Earth's atmosphere and hydrosphere—Earth’s heat engine—are coupled processes that even out solar heating imbalances through evaporation of surface water, rainfall and ocean circulation, when distributing heat around the globe. A Hadley cell is an example of a heat engine, it involves the rising of warm and moist air in the earth's equatorial region and the descent of colder air in the subtropics creating a thermally driven direct circulation, with consequent net production of kinetic energy.

In these cycles and engines, the working fluids are liquids. The engine converts the working fluid from a gas to a liquid, from liquid to gas, or both, generating work from the fluid expansion or compression. Rankine cycle Regenerative cycle Organic Rankine cycle Vapor to liquid cycle Liquid to solid cycle Solid to gas cycle In these cycles and engines the working fluid is always a gas: Carnot cycle Ericsson cycle Stirling cycle Internal combustion engine: Otto cycle Diesel cycle Atkinson cycle Brayton cycle or Joule cycle Ericsson cycle Lenoir cycle Miller cycle In these cyc

The Risk Management Framework is a United States federal government policy and standards to help secure information systems developed by National Institute of Standards and Technology. The two main publications that cover the details of RMF are NIST Special Publication 800-37, "Guide for Applying the Risk Management Framework to Federal Information Systems", NIST Special Publication 800-53, "Security and Privacy Controls for Federal Information Systems and Organizations". NIST Special Publication 800-37, "Guide for Applying the Risk Management Framework to Federal Information Systems", developed by the Joint Task Force Transformation Initiative Working Group, transforms the traditional Certification and Accreditation process into the six-step Risk Management Framework; the Risk Management Framework, illustrated at right, provides a disciplined and structured process that integrates information security and risk management activities into the system development life cycle. The RMF steps include: Categorize the information system and the information processed and transmitted by that system based on an impact analysis.

Vested party is identified. Select an initial set of baseline security controls for the information system based on the security categorization. If any overlays apply to the system it will be added in this step Implement the security controls identified in the Step 2 SELECTION are applied in this step. Assess third party entity assess the controls and verifies that the controls are properly applied to the system. Authorize the information system is granted or denied an Authority to Operate, in some cases it may be postponed while certain items are fixed; the ATO is based off the report from the Assessment phase. Monitor the security controls in the information system are monitored in a pre-planned fashion documented earlier in the process. ATO is good for every 3 years the process needs to be repeated. During its lifecycle, an information system will encounter many types of risk that affect the overall security posture of the system and the security controls that must be implemented; the RMF process supports early resolution of risks.

Risk can be categorized at high level as infrastructure risks, project risks, application risks, information asset risks, business continuity risks, outsourcing risks, external risks and strategic risks. Infrastructure risks focus on the reliability of computers and networking equipment. Project risks focus on budget and system quality. Application risks focus on performance and overall system capacity. Information asset risks focus on the damage, loss or disclosure to an unauthorized part of information assets. Business continuity risks focus on maintaining a reliable system with maximum up-time. Outsourcing risks focus on the impact of 3rd party supplier meeting their requirements. External risks are items outside the information system control that impact the security of the system. Strategic risks focuses on the need of information system functions to align with the business strategy that the system supports. Department of Defense Information Assurance Certification and Accreditation Process, previous program Cyber Risk Quantification NIST Special Publication 800-37 Guide for Applying the Risk Management Framework to Federal Information Systems Risk Management Framework Overview RMF Control Indexer

An egg timer or kitchen timer is a device whose primary function is to assist in timing during cooking. Early designs counted down for a specific period of time; some modern designs can time more by depending on water temperature rather than an absolute time. Traditionally egg timers were small hourglasses and the name has come to be synonymous with this form; as technology progressed mechanical countdown timers were developed which had an adjustable dial and could be applied to a wide range of timed cooking tasks. Most digital timers have been manufactured and a wide selection of software is available to perform this task on a computer or mobile phone; the task is simple to perform on oven timers. New products have been developed which allow for better egg timing; this kind of timer has the potential to more indicate the state of the egg while it is being cooked as they do not rely on certain conditions. One such product is made of translucent plastic with a heat-sensitive coloured disc in the middle which changes colour at 80 °C.

The plastic around the disc changes temperature steadily and from the outside to the inside of the plastic mimicking how an egg heats up while cooking. This allows an observer to see the colour creep inwards through the disc and stop the boiling at the stage required; as it mimics the boiling of an egg, it will be accurate if the boiling process is disrupted, a lower temperature is used and regardless of the quantity of eggs being cooked. Other similar products use electronics to sense the water temperature and play a certain tune or series of beeps to indicate the state of the eggs. Eggs consist of proteins, they become long strands rather than tight masses. They tangle with each other causing the liquid of the egg to become more and more viscous. Most traditional egg timers have a set time of about three minutes, that being the approximate time it takes to cook an average sized hen's egg in water. Hard-boiled eggs take longer to cook; the three minute egg timer is for soft-boiled eggs. The egg changes during the first few minutes of cooking.

The changes cannot be seen through the eggshell, so timing is important. Countdown timers not for eggs are available for general kitchen and timing use. For example, the clockwork Memo Park Timer had a countdown of up to 60 minutes and was sold attached to a keyring, its original purpose being to remind motorists when their parking meter was due to expire