Valve timing optimisation of a spark ignition engine with skip cycle strategy

Abstract

Skip cycle strategy (SCS) is a stroke volume modulation method leading to reduction in pumping loss through deactivation of engine valves under part-load conditions. Although SCS achieves a significant fuel economy, it increases regulated pollutant emissions such as nitrogen oxide and unburned hydrocarbon in comparison to normal 4-cycle engine operation. This paper investigated normal cycle strategy, skip cycle strategy as well as combination of skip cycle strategy and variable valve timing strategy for a spark-ignition engine using one-dimensional numerical model. The skip cycle engine was modelled at several steadystate operation points and then optimised at best ignition timing providing maximum brake torque at each simulation case. The numerical results obtained for both normal cycle and skip cycle have been validated against the experimental data. After completing the validation of numerical results with engine test bench data for both normal and skip cycle operations, optimisation of intake and exhaust valve timing profiles have been carried out regarding advancing or retarding camshaft relatively to the crankshaft position. In case of SCS and variable valve timing application together, NOx concentration was reduced by 35.1%, 39.4%, 26.8% and HC emission was reduced by 54.9%, 49.3% and 47.4% on average for brake mean effective pressure load levels of 1, 2 and 3 bar respectively at all among engine speed ranges between 1200 and 1800 rpm compared to stand alone SCS strategy. Furthermore, no remarkable additional brake specific fuel consumption was observed for SCS plus variable valve timing strategy compared to stand alone SCS.

Increased Internal Combustion Engine Efficiency with Optimized Valve Timings in Extended Stroke Operation

“…There are various methods to exploit operation modes with more than four strokes, for instance as secondary combustion [39], as dual-fuel operation [38], and as heat recovery with water injection [58]. In this paper, we focus on engine cycles with a single combustion event only, as described in [36]. This method is known as skip-cycle or skip-fire strategy, since the additional strokes are used to reduce the combustion frequency, that is, to skip a certain number of combustion events.…”

“…Another method to improve part-load efficiency is the skip-cycle strategy, also known as the skip-fire strategy, as investigated in [36][37][38][39]. With that method, one or more additional engine revolutions are executed after the regular four strokes, which leads to a lower frequency, but an increased load for each combustion instance.…”

“…In [40], a skipped cycle is implemented after one or two consecutive fired cycles, which leads to a brake-specific fuel consumption reduction of 4.3% at a brake mean effective pressure of 1 bar. A reduction of fuel consumption by implementing one skipped cycle after each fired cycle is reported in [36], while a corresponding valve timing adaptation leads to a decrease in emissions. By applying the skip-cycle strategy together with an early intake valve opening, NO x emissions are reduced by up to 40% and, with a late exhaust valve opening, a reduction of HC emissions by up to 55% is achieved.…”

Investigation of cycle skipping methods in an engine converted to positive ignition natural gas engine

“…They indicated that specific fuel consumption reduced as 14% at low partial loads and as 26% at higher partial loads, HC emissions increased in seriously. In another study, Dogru et al 10 carried out a study that they carried out cycle skipping with variable timing strategy as numerically. They achieved a significant change in emissions and especially they achieved a change in NO x emissions to a value of 39.4%.…”

“…In some studies, the cycle skipping strategy was studied by changing the valve timing and minimizing the pumping losses. Dogru et al 10 performed a numerical study that they focused on reducing emissions and they examined the cycle skipping methods with changeable tappet timing. They indicated that NO x emissions improved by approximately 40% at the end of the study.…”