--- title: "Valve timing diagram of four-stroke engine" subject: "Internal Combustion Engines" chapter: 4 key: "semester_6/internal_combustion_engines:valve-timing-diagram-four-stroke" source: https://www.bctengineer.com/q/semester_6/internal_combustion_engines/valve-timing-diagram-four-stroke site: BCT Engineering --- # Valve timing diagram of four-stroke engine Internal Combustion Engines · Chapter 4: Carburetor & Fuel Injection systems - Asked in: year not given, 5 marks - Video: [Valve Timing Diagram For Four Stroke Petrol Engine](https://www.youtube.com/watch?v=hfKTh7GkVME) — BEST MECHANICAL ENGINEERING - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/internal_combustion_engines/valve-timing-diagram-four-stroke - Whole chapter: https://www.bctengineer.com/md/semester_6/internal_combustion_engines/chapter-4 ## Question What is meant by valve timing? Draw a typical valve timing diagram for a four-stroke petrol engine and explain why the valves open before the dead centres and close after them. What is valve overlap? ## Short answer Valve timing is the setting of crank angles for opening and closing inlet and exhaust valves relative to dead centres. Valves open before TDC/BDC and close after to utilise gas inertia for better filling and blowdown for scavenging. Valve overlap is the period near TDC when both valves are open simultaneously, helping clear exhaust and cool the valves. ## Answer **Valve timing** is the setting of the crank angles at which the inlet and exhaust valves open and close with respect to the dead centres. In theory the valves would open and close exactly at TDC and BDC, but this does not work in a real engine. #### Typical diagram (petrol engine, crank angles) ```text TDC (end of exhaust, start of suction) EVC 15 | IVO 10 . - ' ' - -|- - ' ' - . / overlap 25 deg \ | exhaust | suction | | | | \ EVO 40 | IVC 35 / ' - . _ _ _|_ _ _ . - ' BDC (EVO before, IVC after) ``` Angles are in crank degrees: EVC after TDC, IVO before TDC, EVO before BDC, IVC after BDC. | Event | Typical angle | |---|---| | Inlet valve opens (IVO) | 10-20 degrees before TDC | | Inlet valve closes (IVC) | 30-50 degrees after BDC | | Spark | 20-35 degrees before TDC | | Exhaust valve opens (EVO) | 30-50 degrees before BDC | | Exhaust valve closes (EVC) | 10-20 degrees after TDC | (For diesel engines the figures are similar, with injection at about 15-25 degrees before TDC.) #### Reasons - **EVO before BDC:** the gas pressure is still high, so the valve is opened early to let most gas escape (blowdown) and reduce the back pressure on the piston in the exhaust stroke. The small loss in expansion work is less than the gain. - **EVC after TDC:** the moving gas column has inertia, so the cylinder is scavenged more fully. - **IVO before TDC:** so that the valve is fully open when the suction stroke starts. This gives less throttling. - **IVC after BDC:** the fresh charge keeps moving in due to its inertia (ram effect), so cylinder filling is better and the volumetric efficiency is higher. #### Valve overlap The period during which both valves are open together near TDC at the end of exhaust and start of suction (here 25 degrees). It helps scavenging of exhaust gas and cools the valves, but too much overlap lets fresh mixture escape at low speed. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why does the exhaust valve open before BDC? A. Most gas escapes (blowdown), reducing back pressure on the piston in the exhaust stroke B. To admit fresh charge C. To increase the compression ratio D. To cool the piston Answer: A. The small loss of expansion work is less than the gain from lower back pressure. (Key point: Exhaust valve opens early) 2. Why does the inlet valve close after BDC? A. To stop exhaust B. The charge's inertia keeps it flowing in, improving volumetric efficiency C. To start the spark D. To raise exhaust pressure Answer: B. Ram effect of the moving charge fills the cylinder better. (Key point: Inlet valve closes late) 3. What is valve overlap? A. Both valves open near BDC B. Both valves closed at TDC C. Both valves open together near TDC between exhaust and suction D. The time the exhaust valve is open Answer: C. It aids scavenging and valve cooling. (Key point: Valve overlap definition) 4. What is the drawback of too much overlap? A. Lower compression ratio B. Late ignition C. Higher friction only D. Fresh mixture escapes at low speed Answer: D. At low speed, fresh mixture can be pushed out through the open exhaust valve. (Key point: Overlap drawback) ## Related in Internal Combustion Engines - [Two-stroke and four-stroke engine comparison](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Atwo-stroke-versus-four-stroke-engine) — Internal Combustion Engines, chapter 2 - [Four-stroke SI engine working and components](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Afour-stroke-si-engine-working-components) — Internal Combustion Engines, chapter 2 - [IP, BP and efficiencies of a petrol engine](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Aindicated-brake-power-efficiency-numerical) — Internal Combustion Engines, chapter 8 - [Engine displacement and mean piston speed](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Aengine-displacement-piston-speed-numerical) — Internal Combustion Engines, chapter 2 ## Related in other subjects - [Purpose of valves; reflux valve](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Avalves-purpose-reflux) — Water Supply Engineering, chapter 9 - [Purpose of valves; cutoff valve](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Avalves-purpose-cutoff) — Water Supply Engineering, chapter 9 - [Two-stroke versus four-stroke engines](https://www.bctengineer.com/question.md?key=semester_5%2Fautomotive_technology%3Atwo-stroke-vs-four-stroke) — Automotive Technology, chapter 1 - [Engine test: four-cylinder at 1820 rpm](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Aengine-test-1820-rpm) — Power Plant Equipment, chapter 5, asked 5×